Janez Holc

New lead-free relaxors based on the K0.5Na0.5NbO3-SrTiO3 solid solution

New lead-free relaxors have been produced from the K 0.5 Na 0.5 NbO 3 –SrTiO 3 (KNN-STO) system. The solid solubility within the studied range of compositions (1 - x ) K 0.5 Na 0.5 NbO 3 – x SrTiO 3 was observed for x up to 0.33. A pseudo-cubic perovskite structure was determined for x = 0.15 to 0.25. The high density and the uniform distribution of fine grains and pores were confirmed by the translucency of these ceramics. The 0.85KNN-0.15STO composition reaches the dielectric permittivity of above 3000 at room temperature. Dielectric spectroscopy measurements revealed that, as with lead-based complex perovskites, the cationic distribution disorder is reflected in relaxorlike properties, thus suggesting possible applications based on this environmentally friendly lead-free ceramic system.

Influence of the critical point on the electrocaloric response of relaxor ferroelectrics

The electrocaloric effect (ECE), i.e., the conversion of electric energy into heat, is of great importance for application in new generation cooling or heating devices that would be friendlier to the environment. Here, utilizing direct measurements of the ECE change of the temperature ΔT via a high resolution calorimeter, we study the ECE as a function of the magnitude of the electric-field step E in the vicinity of the critical point in several bulk relaxor ferroelectric ceramic systems. Relatively large ΔT of ∼2 to 3 K were obtained at modest fields of 90 kV/cm, even in the case of ceramic materials. The effective responsivity ΔT/E as a function of the electric field shows a characteristic peak near the critical point, which demonstrates the importance of proximity to the critical point for the enhancement of the electrocaloric effect. Experimental results are in good agreement with the theoretical calculations based on the spherical random-bond random-field model.

Electron microscopy studies of potassium sodium niobate ceramics.

Using electron microscopy, K0.5Na0.5NbO3 (KNN) ceramics sintered at 1030 degrees C for 8 h and 1100 degrees C for 2 and 24 h was studied. The scanning electron microscopy and X-ray spectrometry revealed that the materials consisted of a matrix phase in which the (Na+K)/Nb ratio corresponded closely to the nominal composition and a small amount of Nb-rich secondary phase. A bimodal microstructure of cube-shaped grains was revealed in the fracture and thermally-etched surfaces of the KNN. In the ceramics sintered at 1100 degrees C, the larger grains (up to 30 mum across), contained angular trapped pores. The transmission electron microscopy analysis revealed that the crystal planes of the grains bordering the intragranular pore faces were of the {100} family with respect to the simple perovskite cell. Ferroelectric domains were observed in the grains of this material.

Characterisation of LaNi1 − xCoxO3 as a possible SOFC cathode material

The perovskite with nominal composition LaNi0.6Co0.4O3 was evaluated for the use as a possible solid oxide fuel cell (SOFC) cathode. Thick film layers, printed and fired on yttria stabilised zirconia (YSZ) electrolyte, exhibit good characteristics, i.e. a porous microstructure, a temperature expansion coefficient of 11.9 × 10−6/K, and low, temperature independent, sheet resistivity. However, LaNi0.6Co0.4O3 reacts with YSZ at lower temperatures than other perovskites which are usually used for SOFC cathodes.

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 lanthanum gallate based solid electrolyte and ceria

Possible interactions between La{sub 0.9}Sr{sub 0.1}Ga{sub 0.8}Mg{sub 0.2}O{sub 2.85} and Gd{sub 2}O{sub 3}-doped CeO{sub 2} (solid electrolyte and anode binding materials, respectively, for solid oxide fuel cells (SOFC)) at 1,300 C were studied with diffusion couples and fired powder mixtures. The SrLaGa{sub 3}O{sub 7} compound was detected and its formation was attributed to the diffusion of La{sub 2}O{sub 3} from La{sub 0.9}Sr{sub 0.1}Ga{sub 0.8}Mg{sub 0.2}O{sub 2.85} into Ce{sub 1{minus}x}La{sub x}O{sub 2{minus}x/2} solid solution. As the resistivity of SrLaGa{sub 3}O{sub 7} is rather high, around 1 M{center_dot}ohm at 800 C, its presence in the solid electrolyte/anode interface could significantly increase the internal resistivity of an SOFC.

Electron paramagnetic resonance of magnetoelectric Pb(Fe1∕2Nb1∕2)O3

To check on the nature of the weak magnetic order in polycrystalline magnetoelectric Pb(Fe1∕2Nb1∕2)O3 the X-band, Q-band, and far infrared electron paramagnetic resonance (EPR) spectra have been measured between 4 and 600K and compared with magnetic susceptibility and magnetization data. The asymmetric line shapes can be simulated at higher temperature by thermally fluctuating superparamagnetic nanoclusters. The pronounced temperature dependence of the position of the spectra demonstrates the presence of an internal magnetic field which is small but nonzero even at room temperature, i.e., far above the antiferromagnetic transition. The electronic spin-spin exchange has been found to be in the terahertz range. The magnetization data reveal a weak ferromagnetism even above 300K and a break in the temperature dependence of susceptibility at the paramagnetic to ferromagnetic transition.

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.

High-frequency transducers based on integrated piezoelectric thick films for medical imaging

A screen-printed PZT thick film with a final thickness of about 40 mum was deposited on a porous PZT substrate to obtain an integrated structure for ultrasonic transducer applications. This process makes it possible to decrease the number of steps in the fabrication of high-frequency, single-element transducers. The porous PZT substrates allow high acoustic impedance and attenuation to be obtained, satisfying transducer backing requirements for medical imaging. The piezoelectric thick films deliver high electromechanical performance, comparable to that of standard bulk ceramics (thickness coupling factor over 45%). Based on these structures, high-frequency transducers with a center frequency of about 25 MHz were produced and characterized. As a result, good sensitivity and axial resolution were obtained in comparison with similar transducers integrating a lead titanate (PT) disk as active material. The two transducers were integrated into a high-frequency imaging system, and comparative skin images are shown

TEMPERATURE CHARACTERISTICS OF ELECTRICAL PROPERTIES OF (BA,SR) TIO3 THICK FILM HUMIDITY SENSORS

Abstract Ceramic materials based on perovskite-type oxides can be used as humidity sensors. Thick film sensors based on (Ba0.5Sr0.5TiO3 material doped with MgO were prepared by printing and firing thick film pastes on 96% alumina substrates. Fritless platinum paste was used as the electrode material. The a.c. respone of the thick film humidity sensors at 20–80% relative humidity and 10–60 °C was measured. The conductance and susceptance of the sensors versus relative humidity were measured at 400 Hz. The capacitive component of the sensors impedance at low humidity (20–4-% relative humidity) has negative temperature characteristics and obeys the Curie-Weiss law. This indicates the dielectrical properties of the adsorbed water on the perovskite sensor surface. The conductance is logarithmically linear. At high humidity a positive temperature characteristic of the capacitive component was observed. An inversion between these two characteristics was detected at about 40% relative humidity for all measured temperatures.