Zoran Samardžija

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.

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.

Characterisation of thick film resistor series for strain sensors

Abstract Some 10 kohm/sq. thick film resistors based on RuO 2 , ruthenates or a mixture of RuO 2 and ruthenates, were evaluated for strain gauge applications. The resistors were fired at different temperatures to estimate the influence of firing temperature on the electrical characteristics. Temperature coefficients of resistivity (TCR), noise indices and gauge factors (GF) were measured. Microstructures of the thick film resistors were analysed by SEM. The results indicate that the microstructure of thick film resistors influences the gauge factors much more significantly than the “nature” of the conductive phase.

The development of microstructural and electrical characteristics in some thick-film resistors during firing

The development of microstructural and electrical characteristics (sheet resistivities, TCRs, and noise indices) in some thick-film resistors during the firing process has been evaluated. Three 1 kohm/sq. resistor pastes (Du Pont), based on RuO2, ruthenate or a mixture of both conductive phases, were fired at temperatures from 500°C to 950°C. The cell parameters of the RuO2 in the 8031 and the 2031 resistors, the bismuth ruthenate in the 8029 resistors and the lead ruthenate in the 2031 resistors, were calculated from X-ray data. Microstructures were analyzed by SEM and EDS microanalysis. The absolute values of the cold and hot TCRs first decreased to a minimum at 850°C and then increased again with increasing firing temperature. The noise indices of the resistors generally decrease with increasing firing temperature.

Phase equilibria in the BaTiO 3 –La 2 TiO 5 –TiO 2 system

Subsolidus phase relations in the BaTiO 3 –La 2 TiO 5 –TiO 2 part of the ternary BaO–La 2 O 3 –TiO 2 system at 1300 °C in air were determined. The phases were characterized by x-ray diffraction, scanning electron microscopy, and electron probe wavelength dispersive spectroscopic microanalysis. A combination of techniques was employed because of insensitivity in detecting secondary phases by x-ray diffraction. The location and extent of Ba 6− x La 8+2 x /3 Ti 18 O 54 ternary solid solution 0.2(1) ⩽ x ⩽ 2.3(1) and Ba 1− y La y Ti 1− y /4 (V Ti ) y /4 O 3 binary solid solution 0 ⩽ y ⩽ 0.3 at 1300 °C was established. Tie lines between various barium polytitanates with a sequence of Ba 6− x La 8+2 x /3 Ti 18 O 54 solid solution regions were determined.

Temperature sensors made by combinations of some standard thick film materials

Thick film resistors consist basically of a conductor phase, a glass phase and an organic vehicle, which burns out during high temperature processing. In most contemporary resistor compositions the conductive phase is either RuO2 or ruthenates. In addition, some other oxides are added either as temperature coefficient of resistivity (TCR) modifiers or modifiers of the temperature coefficient of expansion (TEC) of glass phase [1–3]. Most thick film resistor materials are designed for printing and firing on alumina substrates. The main requirements for their characteristics are stability, relatively narrow tolerances of sheet resistivities, and a low temperature coefficient of resistivity (TCR). During firing the resistors are at the highest temperature (typically 850◦C) a relatively short time (typically 10 min). All the constituents of the resistor material react with each other, with the conductor termination and also with the substrate [4–6]. Due to the short time at firing temperature the reactions do not reach equilibrium so that the characteristics of fired materials are, to an extent, a compromise as the consequence of this frozen unequilibrium. If thick film resistors, intended for firing on alumina substrates, interact with other materials, for example with multilayer dielectrics or NTC materials (thick film resistors with high negative and nonlinear TCRs) this could influence the resistors’ characteristics, in most cases for the worse. This is probably due to the interaction between different materials although some authors ascribe this to a different TEC [6–9]. As already mentioned the TCR of thick film resistors is aimed to be as low as possible. However, some combinations of thick film materials have been reported to result in characteristics, which allow them to be used for some temperature sensor applications (7, 10, 11). To make a useful sensor the dependence of resistivity vs. temperature ought to be high enough (high TCR) and reasonably linear. Some combinations of resistor materials and multilayer dielectrics or NTC resistors resulted in these desired characteristics. This indicates a possible use of standard thick film pastes for making fairly useful and inexpensive temperature sensors. Note, however, that data reported in this letter are not the result of a systematic search for such combinations and that the mechanisms responsible for such “behavior” were not investigated. The thick film materials, which were used in different combinations, are listed in Table I. The nominal sheet resistivities of the resistors and NTC thermistors are 1 and 10 kohm/sq. For comparison, the PTC resistor (high positive linear dependence of resistivity vs. temperature) and the platinum based conductor are also included, but note that these two materials were not fired in combination with other materials. Thick film resistors (sheet resistivities 1 kohm/sq. and 10 kohm/sq.) are part of the Du Pont HS-80 resistor series. However, the materials denoted 80× 1 and 80× 9 (for example, 1 kohm/sq. resistors 8031 and 8029) differ in the conductive phase used as well as in the fired resistor microstructure. In order to observe the microstructure the fired resistors were mounted in epoxy in a cross-sectional orientation and then polished using standard metallographic techniques. Prior to analysis in the scanning electron microscope (SEM), the sample was coated with carbon to provide electrical conductivity and to avoid charging effects. A JEOL JSM 5800 SEM equipped with an energy dispersive X-ray analyser (EDS) was used for overall microstructural and compositional analysis.

The extent of solid solubility in the RuO 2 –TiO 2 system

RuO 2 single crystals were obtained by evaporation of PbO from Pb 2 Ru 2 O 6.5 at high temperatures and were verified as good standards for WDS analysis. They were used for the investigation of phase equilibria and the extent of solid solubility in the RuO 2 –TiO 2 system by WDS quantitative microanalysis. The solid solubility at 1350 °C was determined to be 16.5% TiO 2 in RuO 2 and 13.5% RuO 2 in TiO 2 .

Molecular ultrastructure of the urothelial surface: insights from a combination of various microscopic techniques.

The urothelium forms the blood–urine barrier, which depends on the complex organization of transmembrane proteins, uroplakins, in the apical plasma membrane of umbrella cells. Uroplakins compose 16 nm intramembrane particles, which are assembled into urothelial plaques. Here we present an integrated survey on the molecular ultrastructure of urothelial plaques in normal umbrella cells with advanced microscopic techniques. We analyzed the ultrastructure and performed measurements of urothelial plaques in the normal mouse urothelium. We used field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), transmission electron microscopy (TEM) on immunolabeled ultrathin sections (immuno‐TEM), and freeze‐fracture replicas (FRIL). We performed immunolabeling of uroplakins for scanning electron microscopy (immuno‐FESEM). All microscopic techniques revealed a variability of urothelial plaque diameters ranging from 332 to 1179 nm. All immunolabeling techniques confirmed the presence of uroplakins in urothelial plaques. FRIL showed the association of uroplakins with 16 nm intramembrane particles and their organization into plaques. Using different microscopic techniques and applied qualitative and quantitative evaluation, new insights into the urothelial apical surface molecular ultrastructure have emerged and may hopefully provide a timely impulse for many ongoing studies. The combination of various microscopic techniques used in this study shows how these techniques complement one another. The described advantages and disadvantages of each technique should be considered for future studies of molecular and structural membrane specializations in other cells and tissues. Microsc. Res. Tech. 77:896–901, 2014.

Metastable quasicrystals in Al–Mn alloys containing copper, magnesium and silicon

We prepared three Al–Mn-based alloys with different copper, magnesium and silicon contents by casting into cylindrical copper molds. All the alloys exhibited primary metastable quasicrystals (QCs). In order to confirm the presence of either primary decagonal QCs (dQCs) or icosahedral QCs (iQCs) and to determine their compositions, the castings were characterized by means of light microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy (EDS), electron-backscatter diffraction and X-ray diffraction. The dQCs are present in the Al–Mn-based alloys containing copper. In the case of the combined presence of copper and magnesium, iQCs are present in the edge region and dQCs are present in the central region. In the alloy containing copper, magnesium and silicon, iQCs are present in the casting. The average metallic radius (AMR) and electron-to-atom ratio of these primary phases were calculated by taking into account the composition of these primary phases, as determined by EDS. The AMR shows different values in the cases of dQCs and iQCs. Equal mean values of the AMR were found in iQCs with markedly different compositions. Furthermore, all the metastable QCs in this work show electron concentrations close to 2.6.

Phase equilibria in the RuO2-Bi2O3-NiO and RuO2-Al2O3-NiO systems

After firing, thick film resistors consist of a conductive phase in a glass matrix. The conductive phase is either RuO2 or ruthenates, usually Bi2Ru207 [1, 2]. Specific conductivities of RuO2 and Bi2Ru207 are 40 × 10 .6 ohm cm and 150 x 10 .6 ohm cm, respectively [3, 4]. However, resistors prepared from a mixture of ruthenium oxide or ruthenates and a glass phase have relatively high positive temperature resistivity coefficients (TRC). To decrease the TCR, small amounts of so-called TCR modifiers, i.e. semiconducting oxides with negative TCR, are added. Among many others, NiO is reported to be sometimes used as a TCR modifier [5]. Thick film circuits are made by printing and firing thick film pastes on ceramic substrates. The ceramic mainly used is A1203. During firing, alumina dissolves in the glass and could react with components of the thick film resistors [6, 7]. The aim of this work was to investigate phase equilibria in the RuO2-Bi203-NiO and RuO2NiO-A12Oa systems. Both ternary systems imply possible interactions which can occur between the conductive phase in the resistors and NiO on the one hand, and the alumina substrate on the other. The RuO2-Bi203, RuO2-A1203 and RuO2-NiO systems have been investigated by Hrovat et al. In the RuO2-BizO3 system the binary compound Bi2Ru207 decomposes at temperatures over 1200 °C into RuO2 and Bi203. The melting point of the eutectic (80% Bi203) is 745 °C [8]. In the RuO2-A1203 [9] and RuO2-NiO [10] systems there is no binary compound and no liquid phase (eutectic) up to 1400 °C, the temperature at which RuO2 decomposes in air to metallic ruthenium and oxygen. A solid solution between Bi2Ru207 and NiO with the formula Bi2_xNixRu2OT_x/2 for x between 0 and 0.4 was reported by Schuler and Kemmler-Sack [11]. The Bi203-NiO system on the Bi203-rich side (to 24% NiO) was investigated by Levin and Roth [12]. The eutectic temperature is around 810 °C while the eutectic composition was not determined. The A1203-NiO system was studied by Phillips et al. [13]. The binary compound (spinel) NiA1204 melts congruently at 2100 °C. The melting point of the eutectic with a lower melting temperature on the NiO-rich side (85% NiO) is 1870 °C. For experimental work, RuO2 (Ventron, 99.9%), Bi203 (Merck, 99.9%), NiO (Riedel de Haen, +99.9%) and A1203 (Alcoa, A-16)were used. The samples were mixed in ethyl alcohol, pressed into pellets and fired with intermediate grinding. During firing, pellets were placed on platinum foils. The compositions of the relevant samples are shown in Figs 3 and 4. The results were evaluated by X-ray powder analysis, differential thermal analysis, scanning microscopy and EDS (energy dispersive X-ray microanalysis). The eutectic temperature and composition were determined using a standard DTA procedure. The area of the peaks on the DTA curves was measured for samples with different compositions and the eutectic composition was extrapolated from these data. The proposed nickel oxide-bismuth oxide phase diagram is shown in Fig. 1. There is no binary compound. The microstructure of a polished NiO/ Bi203 (80% Bi203 and 20% NiO) sample, fired at 780 °C, is presented in Fig. 2. The material is a mixture of darker NiO grains in a Bi203 matrix. The eutectic composition is around 95% Bi203 and the eutectic temperature approximately 795 °C, some 10 K to 15 K lower than reported in [12]. The body centred cubic phase (NiO stabilized 7-Bi203) with a