Effect of alumina additions in YSZ on the microstructure and degradation of the LSM–YSZ interface

The cathode–electrolyte interface in a solid oxide fuel cell is examined to understand why premature delamination is observed in alumina substituted YSZ electrolyte. From XRD, SEM and TEM observations it was concluded that after high temperature sintering a tetragonal (Mn,Al)₃O₄ forms at the interface, which during prolonged fuel cell operation forms a cubic (Mn,Al)₃O₄ phase. This transformation is associated with volume decrease creating voids which ultimately weaken the cathode–electrolyte interface sufficiently for the cathode layer to delaminate off the YSZ–Al₂O₃ electrolyte.     

TPD study in LSM/YSZ/LaAlO system for the use of fuel cell type reactor

Behaviors of oxygen species in the LSM/YSZ/LaAlO SOFC type reactor were investigated. The fuel cell type temperature-programmed desorption (FC-TPD) measurements were employed in three operating modes; i.e. (1) open circuit, (2) closed circuit without applied potential and (3) closed circuit with applied potential modes. Various pretreatment conditions, i.e. (a) open circuit, (b) closed circuit without applied potential and (c) closed circuit with applied potential, were investigated. The FC-TPD technique enables to discuss the oxygen species responsible for a combustion reaction and those for a coupling reaction, separately. The open circuit type FC-TPD showed that the applied potential during the pretreatment caused the increase in the amount of surface oxygen and the decrease in activation energy of desorption of oxygen at the anode catalyst. The closed circuit FC-TPD provided the combined effects of desorption and permeation. In addition, the FC-TPD analysis could correlate the behavior of adsorbed oxygen species to the NEMCA effect observed in the oxidative coupling of methane in the SOFC reactor.     

Electrode reaction at platinum / ceria surface modified YSZ interface

The electrode reaction was examined on ceria coated YSZ by a platinum point electrode in H₂-H₂O atmosphere at 973 K- 1173 K. The thickness of the ceria coating layer was altered from 0 to 2.5 μm, fabricated by a laser ablation and by a vacuum vapor deposition method on YSZ single crystals. The electrode / electrolyte interface conductivity increased with 1/4 powers ofp(H₂) andp(H₂O) on both ceria coated and non-coated YSZ. The interface conductivity was significantly improved on a thicker ceria coating surface than 1 μm. The effective electrode reaction radius also increased in a thick ceria coating. The¹⁸O/¹⁶O exchange experiment at low oxygen partial pressure revealed that the oxygen surface exchange rate of ceria is not high compared with that of YSZ. It can be concluded that the bulk ionic conduction of ceria makes a more effective contribution to the electrode reaction than the surface catalytic activity in H₂-H₂O atmosphere. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Origin of cathodic degradation and new phase formation at the La0.9Sr0.1MnO3/YSZ interface

New phase formation at the La_0.9Sr_0.1MnO₃/YSZ interface and its effects on the cathodic performances were studied at 900 °C in air. The resistance caused by the interfacial product layer kept increasing with time to reach up to 40% of the total resistance after 500 h. The interfacial product was identified as La₂Zr₂O₇ by XRD measurement. The electrical conductivity of La₂Zr₂O₇ (2.4 × 10⁻⁵ S cm⁻¹ at 1000 °C), measured by AC impedance and current interruption methods, was 4 to 7 orders of magnitude smaller than those of La_0.9Sr_0.1MnO₃ electrode or YSZ electrolyte. Either the electronic conductivity or the electrochemical O₂ reduction activity of La₂Zr₂O₇ was negligible. Combining these results, a conclusion was made that the cathodic degradation comes mainly from the growth of interfacial product layer and its contribution to the cell resistance increment is ohmic in nature.     

Conductivity degradation of NiO-containing 8YSZ and 10YSZ electrolyte during reduction

Yttria-stabilized zirconia (YSZ) electrolyte containing dissolved nickel as a result of co-sintering in air with NiO was shown by TEM and magnetic measurements to develop isolated, nano-sized metallic nickel precipitates during treatments in hydrogen-containing atmosphere. During exposure to hydrogen containing atmosphere at 1000 °C for 500 h, an exponential decrease in oxygen ion conductivity was observed in Ni-doped 8YSZ, while no conductivity change was observed in Ni-doped 10YSZ.     

Electrical conductivity of thick film YSZ

When yttria-stabilized zirconia (YSZ) electrolyte is coated and co-sintered on top of Ni–YSZ anode support, the measured conductivities of YSZ thick films (10–30 μm thick) are often lower than that of bulk YSZ. In this study, we found the observation by fabricating free-standing YSZ thick films and measuring and comparing in-plain and across-plain conductivities. The in-plane conductivity of free-standing YSZ film matched very well with the conductivity of mm-thick bulk sample. It was further shown that the conductivity decrease can be minimized by using better electrode morphology.Another factor that decreases the film conductivity was identified when the thick film was reduced. The conductivity decrease, ∼26% after reduction for 1h in humidified hydrogen gas, was due to Ni-doping into YSZ during sintering process.In order to minimize the conductivity drop of thick film YSZ during SOFC (solid oxide fuel cell) operation, an intermediate layer may be used between YSZ and anode support to prevent Ni-doping during co-sintering process in addition to the well-designed electrode morphology.     

Electrical behaviour of LSGM–LSM composite cathode materials

Composite cathode materials were prepared by mixing La_0.83Sr_0.17Ga_0.83Mg_0.17O_2.83 (LSGM) and La_0.8Sr_0.2MnO₃ (LSM) powders fired at 1300 °C. Several compositions were set up containing 1, 5, 25, 50, 75 wt.% of LSM. Their microstructure and electrical behaviour were investigated by XRPD, SEM/EDS and EIS. In composites containing 50 and 75 wt.% of LSM, the electronic contribution to conductivity is predominant, then there is only a single point at the low frequency end of the Nyquist plot. On the contrary, in the composites with up to 25 wt.% of LSM, there is a significant amount of ionic transport, then the IS spectra show complex features: at least three different arcs can be devised and their interpretation depends upon temperature. LSGM bulk and grain boundary conductivity, as well as interface polarization between the ionic (LSGM) and electronic (LSM) phases can be separated at temperatures below 600 °C; total LSGM contribution, i.e. bulk plus grain boundary, LSGM–LSM interface and electrode polarizations are attributed above 600 °C.     

Electrical characterization of highly Titania doped YSZ

The defect fluorite region of the ternary system ZrO₂-Y₂O₃-TiO₂ encompasses compositions which offer both, good electronic and oxygen ion conductivity which enable good catalytic activity for the direct oxidation of methane in a solid oxide fuel cell (SOFC). The electrical properties of compositions Y_xTi_yZr_1−(x+y)O_2−x/2 (with x=0.15, 0.2, 0.25 and y=0.15, 0.18) were characterised in order to find the composition with highest ionic and electronic conductivity. High titanium dopant concentrations (Y) of 15 and 18 atom%, near the solubility limit of Ti⁴⁺ in the fluorite structure, have been introduced to achieve a high electronic conductivity at low oxygen partial pressure. The yttrium content x has been varied between 15 and 25 atom% to find the fluorite composition with the highest ionic conductivity for each titanium level. In the pO₂-range from 0.21 to 10⁻¹³ atm the conductivity is predominantly ionic and constant over that range. The maximum ionic conductivity is 0.01 Scm⁻¹ for the compositions, which contain 15 atom% yttrium. Substantial electronic conductivity is introduced into the system at low oxygen pressures below 10⁻¹³ atm via reduction of Ti⁴⁺ ions to Ti³⁺. The maximum electronic conductivity of 0.2 Scm⁻¹ at 930 °C has been measured for a sample with 18 atom% titanium. The slope of all log(σ) vs. log(pO₂) plots follows a pO ₂ ^−1/4 -dependence. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Behaviour of a dense In2O3/ YSZ electrode in oxygen containing atmospheres

A study of electrical and electrochemical properties of a dense In₂O₃ electrode in contact with a single crystal YSZ electrolyte was carried out by d.c. and a.c. methods. As a result, it was found that dense In₂O₃ electrodes have high electrical conductivity but very low electrochemical activity. In a vicinity of the equilibrium potential and under the anodic polarisation, the rate of Faraday reaction at the In₂O₃ electrodes was as low as to consider the electrode a blocking one. The blocking properties of the In₂O₃ electrodes were used to measure the hole conductivity of the YSZ electrolyte in the temperature range between 795 to 1163 K and oxygen partial pressure from 1 to 10⁵ Pa. A comparison with the literature data confirmed that the dense In₂O₃ electrode blockes the ionic transfer through the YSZ. A set of experiments indicated that the oxygen exchange between the indium oxide surface presented to the oxygen containing gaseous phase and this phase is very poor. A route of the electrode process at O₂, In₂O₃ / YSZ electrode was proposed a limiting stage of which is the discharge of the oxygen ions to the atomic oxygen adsorbed on the electrode surface: $$O_0 ^x \left( {In_2 O_3 } \right)_s = V_0 ^{ \bullet \bullet } \left( {In_2 O_3 } \right)_s + O_{ad} \left( {In_2 O_3 } \right)_s + 2e'\left( {In_2 O_3 } \right).$$ The polarisation resistance decreases when platinum or the praseodymium oxide is deposited on the surface of the In₂O₃ electrodes. The cathodic polarisation also increases the electrochemical activity of the electrodes. Both the establishment of the steady state of the electrode under polarisation and the recovery of the equilibrium state by the electrode are very long processes, which are probably related to the diffusion mechanism by which the stoichiometry of the indium oxide is changed.     

Electrochemical characterisation of the Pt/YSZ interface exposed to a reactive gas phase

The Pt/yttria-stabilized cubic zirconia (YSZ) interface exposed to a reactive gas was characterised by solid electrolyte potentiometry and cyclic voltammetry. The catalytic reactions included total combustion of C₃H₈ and C₃H₆ to CO₂ and H₂O as well as NO reduction by C₃H₆ in the presence of O₂ under oxygen-rich and stoichiometric conditions. The solid electrolyte potentiometry as a function of the temperature in C₃H_x/O₂ (with x=6 or 8) reflected the catalytic properties of Pt for C₃H_x oxidation. In C₃H₆/NO/O₂, the reduction of NO was evidenced below 300 °C. The cyclic voltammetry evidenced the formation of an oxygen chemisorbed layer on the Pt surface under anodic potential. Propane had no effect on this chemisorbed layer, whereas propene weakened significantly the strength of this Pt–O bond. Addition of NO to C₃H₆/O₂ led to the disappearing of this chemisorbed layer. The use of solid electrolyte potentiometry in conjunction with cyclic voltammetry allowed us to determine the surface oxidation state of Pt during the catalytic reactions.     

Fabrication of porous Ni–YSZ anodes by PSH-PIM

This paper has investigated the fabrication process of porous Ni–YSZ anodes by the powder injection molding method, in which a powder space holder (PSH) is used. Polymethyl methacrylate (PMMA) has been used as a PSH for mixing with NiO–YSZ powders. For this study, five kinds of feedstock containing 0%, 10%, 20%, 30%, and 40% PMMA by volume were prepared. The thermoplastic binder used for the process had a fixed 35 vol.%, and the powder loads formed the remaining 65, 55, 45, 35, and 25 vol.% of the feedstock. After molding and debinding, the parts were sintered at 1,500 °C. The obtained results showed that increasing the PMMA portion of the feedstock and reducing its powder load causes the viscosity of the feedstock to decrease. The amount of shrinkage of the samples containing 0–30% PMMA showed an almost linear increase with the increase of the PMMA content, and for the samples with 40% PMMA, this increase of shrinkage was higher. The amount of porosity in the samples having 0–30% PMMA increased with the rise in the PMMA content, but in the samples containing 40% PMMA, the amount of porosity decreased, such that it was less than that of the samples with 30% PMMA. The electrical conductivity and flexural strength of all the samples were also studied in this work.     

Electrical conductivity of YSZ film grown by pulsed laser deposition

Yttria-stabilized zirconia (YSZ) thin films, 0.6–1.5 μm, were deposited on Pt and sapphire substrates by a pulsed laser deposition (PLD) method. Their structural and transport properties have been studied by means of X-ray diffraction and electrical conductivity measurements. The in-plane and the perpendicular-to-plane conductivities (hereafter, “across-plane” conductivity) of thin films were measured and compared to that of bulk sample. X-ray diffraction and electron microscopy results showed that the films on Pt and sapphire were polycrystalline cubic with a columnar structure. Both the across-plane and the in-plane conductivities of YSZ thin film were close to that of bulk specimens. Thus no conductivity enhancement was found for the present nano-crystalline YSZ films (grain or column size, 60∼100 nm).     

Finite element modelling of dense and porous piezoceramic disc hydrophones

The acoustic characteristics of dense and porous piezoceramic disc hydrophones have been studied by finite element modelling (FEM). The FEM results are validated initially by an analytical model for a simple disc of dense piezoceramic material and then it is extended to a porous piezoceramic disc replicating a foam-reticulated sample. Axisymmetric model was used for dense piezoceramic hydrophone due its regular geometric shape. 3-dimensional model was used for the porous piezoceramics, since the unit cell model is inadequate to fully represent transducers of finite lateral dimensions. The porous PZT discs have been synthesised by foam-reticulation technique. The electrical impedance and the receiving sensitivity of the hydrophones in water are evaluated in the frequency range 10-100 kHz. The model results are compared with the experimental data. The receiving sensitivity of piezocomposite hydrophones is found to be reasonably constant over the frequency range studied. The sharp resonance peaks observed for the dense piezoceramic hydrophone has broadened to a large extent for porous piezoceramic hydrophones, indicating higher losses. The flat frequency response suggests that the 3-3 piezocomposites are useful for wide-band hydrophone applications.     

Electrical characteristics of the rf-excited oxygen plasma-cathodization-grown SiO2/Si interface

The electrical properties of the SiO₂/n-type Si(100) interface, where the silicon-oxide layer was grown by an electrodeless rf oxygen-plasma-cathodization technique, were investigated usingC-V and DLTS methods. Interface traps with high density in the range of 10¹² eV^–1 cm^–2 and a capture cross section as large as 10^–18 cm² were found in the upper region of the silicon forbidden gap. After a post-annealing process, typically at 400°C for 30 min in dry N₂ atmosphere, their densities and capture cross sections were reduced to the range of 1–2 × 10¹¹ eV^–1 cm^–2 and 10^–19 cm^–2, respectively. Apparant differences in DLTS curves before and after thermal annealing were also observed. Results are qualitatively explained by considering the specific oxidation and annealing mechanism of this low-temperature silicon-oxidation technique.     

Non-faradaic electrochemical modification of the catalytic activity for propane combustion of Pt/YSZ and Rh/YSZ catalyst-electrodes

The effect of non-faradaic electrochemical modification of catalytic activity (NEMCA effect) or electrochemical promotion (EP) was investigated in the total oxidation of propane on porous Pt and Rh catalyst-electrode films interfaced to 8 mol% Y₂O₃-stabilized-ZrO₂ (or YSZ), in the temperature range 425–520 °C and for sub-stoichiometric O₂ to propane ratios. Application of either positive or negative overpotentials resulted in non-faradaic increase of the catalytic rate, by up to a factor of 4 in the case of Rh and by up to a factor of 1350 in the case of Pt. The rate increase observed in the case of Pt is among the highest ones reported so far in NEMCA studies with oxygen ion conductors as active supports.     

金属/多孔硅/单晶硅(M/PS/Si)微结构的电学特性

采用双槽电化学腐蚀法制备多孔硅层,然后在多孔硅表面沉积形成金属电极,制备出M/PS/Si微结构.利用SEM分析多孔硅的表面形貌,通过测试其I-V特性分析M/PS/Si微结构的电学特性.结果表明:由Pt做电极形成的M/PS/Si结构,表现出非整流特性.M/PS/Si结构的I-V曲线由线性区和非线性区组成,多孔硅孔隙率越高的M/PS/Si结构的I-V特性曲线线性区越宽.由Cu做电极形成的M/PS/Si结构,表现出整流特性.其整流比随多孔硅孔隙率增加而减小. 关键词： M/PS/Si微结构 孔隙率 I-V特性')" href="#">I-V特性 欧姆接触     

Electrical conductivity of NaCl ¦ graphite interface

It was observed that a steady d. c. current flows through the cell graphite ¦NaCl¦ graphite even at voltages very small compared to the decomposition potential difference. The activation enthalpy of the small signal conductance (1·20±0·05) eV differs from the enthalpy of the bulk conductivity. The interface conductance is neither in simple relation to the conductivity of the crystal, nor to the partial conductivities of anions and cations. The nonlinearity of the cell and the independence of interface conductance from some properties of the bulk can be explained by the finite rate of electrochemical reactions, ensuring the charge transfer across the interfaces.     

Nanostructuring Si surface and Si/SiO2 interface using porous-alumina-on-Si template technology. Electrical characterization of Si/SiO2 interface

In this work, anodic porous alumina thin films with pores in the nanometer range are grown on silicon by electrochemistry and are used as masking material for the nanopatterning of the silicon substrate. The pore diameter and density are controlled by the electrochemical process. Through the pores of the alumina film chemical oxidation of the silicon substrate is performed, leading to the formation of regular arrays of well-separated stoichiometric silicon dioxide nanodots on silicon, with a density following the alumina pores density and a diameter adjustable by adjusting the chemical oxidation time. The alumina film is dissolved chemically after the SiO₂ nanodots growth, revealing the arrays of silicon dioxide dots on silicon. In a next step, the nanodots are also removed, leaving a nanopatterned bare silicon surface with regular arrays of nanopits at the footprint of each nanodot. This silicon surface structuring finds interesting applications in nanoelectronics. One such application is in silicon nanocrystals memories, where the structuring of the oxidized silicon surface leads to the growth of discrete silicon nanocrystals of uniform size. In this work, we examine the electrical quality of the Si/SiO₂ interface of a nanostructured oxidized silicon surface fabricated as above and we find that it is appropriate for electronic applications (an interface trap density below 1–3×10¹⁰ eV⁻¹ cm⁻² is obtained, indicative of the high quality of the thermal silicon oxide).     

Electrical characterization of surface and interface potentials on SiC

Contact free vibrating capacitor results have shown that the SiC surface is more stable, compared to Si, and it is possible to identify the different (Si or C) planes on SiC substrates. The surface charge density seems to be higher after compression welding process. Electrostatic (corona) charge on the surface results in accumulation and depletion, and probably avalanche breakdown instead of equilibrium inversion. However, the equilibrium Q–V curve still can be measured starting from the inversion region.Among C–V methods the capabilities of V–Q and mercury C–V have been investigated, as two major electrical measurement techniques for SiC qualification. SiC–silicon-dioxide interfaces and SiC epitaxial layers were characterized with HF/LF C–V and V–Q measurement techniques. These methods were developed basically for Si measurements, but they could easily be adapted for measuring SiC too.