Marko Hrovat

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.

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.

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.

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.

Interactions between alumina and PLZT thick films

For some applications, piezoelectric (Pb,La)(Ti,Zr)O3 solid solution (PLZT) thick films are screen-printed and fired on platinum-metallized Al2O3 substrates. In this work, the compatibility and adhesion between PLZT thick films, the thick film platinum electrode, and the alumina substrate was investigated. Microanalysis of PLZT/Pt/Al2O3 structures after the firing process showed a secondary phase with β-Al2O3 structure at the interface between Pt and Al2O3, with a nominal composition of PbAl12O19, doped with La2O3 and TiO2 from PLZT and partly with oxides originating from impurities in alumina (CaO and MgO). As this compound could not be synthesized from pure oxides the results indicate that the other oxides are needed to promote the formation of a β-Al2O3 type compound. The formation of this compound results in significantly decreased adhesion between the PLZT/Pt layer and the alumina substrate.

Relaxorlike dielectric dynamics in the lead-free K0.5Na0.5NbO3-SrZrO3 ceramic system

We have detected a typical relaxor/glasslike dielectric response in the derived lead-free (1−x)K0.5Na0.5NbO3-(x)SrZrO3 ceramic system. In the compositional range 0.10≤x≤0.33, where solid solubility was observed, the system exhibits a broad dispersive dielectric maximum, a Vogel–Fulcher temperature dependence of the characteristic relaxation frequency, and a paraelectric-to-glass crossover in the temperature dependence of the dielectric nonlinearity. As the relatively high values of the dielectric constant, exceeding 5000 in the x=0.15 compound, are only weakly dependent on the frequency in the range of 20 Hz–1 MHz, and, furthermore, the history-dependent effects play only a minor role in comparison to some widely used lead-based relaxors, this environmentally friendly system seems to be very promising for a variety of applications.

Subsolidus phase equilibria in the NiO–CeO2 and La2O3–CeO2–Fe2O3 systems

Subsolidus equilibria in air in the NiO–CeO2 and La2O3–CeO2–Fe2O3 systems were studied. In the NiO–CeO2 and Fe2O3–CeO2 systems, no binary compound and no solid solubility were detected. La2O3 was soluble in CeO2, forming a cubic fluorite solid solution up to La0.5Ce0.5O1.75, whereas no solid solubility of CeO2 in La2O3 was detected. In the La2O3–CeO2–Fe2O3 system, no ternary compound was found. The tie line is between LaFeO3 and CeO2.

A characterisation of thick film resistors for strain gauge applications

Some commercial thick film resistors with sheet resistivities from 1 kohm/sq. up to 1 Mohm/sq. were evaluated for strain gauge applications. Temperature coefficients of resistivity, noise indices and gauge factors (GFs) were measured. For the same resistor series GFs and noise indices increase with increasing sheet resistivity. However, both GFs and noise indices are different for resistors with the same nominal sheet resistivity but from different resistor series. The results indicated that the microstructure rather than the different chemical composition of the conductive phase in thick film resistors is the primary reason for the different gauge factors.