Resistivity relaxation, a new approach to study ionic mobility in perovskite mixed conductors like CaTi0.7Fe0.3O3–δ

Oxygen transport of mixed ionic-electronic conductors can be measured by a ‘relaxation’ technique that permits to investigate the material dynamic properties with oxygen partial pressure change. However, for materials exhibiting higher electronic conductivity than ionic, the time for conductivity change is controlled by bulk ionic transport and any surface reaction can be neglected. By fitting the experimental relaxation data of CaTi_0.7Fe_0.3O_3–δ composition, the oxidation and reduction kinetics was found to be independent on oxygen partial pressure ( ) and the rate constants were derived therefrom. From a relaxation experiment at a single we therefore obtain both the electronic and ionic contributions to the total conductivity as well as the chemical diffusion coefficient.     

Thermal stability, oxygen non-stoichiometry and transport properties of LaNi0.6Fe0.4O3

Thermal stability, oxygen non-stoichiometry and electrical conductivity of LaNi_0.6Fe_0.4O_3−δ were investigated in the temperature region of 20-1000 °C in Ar/O₂ gas flows at oxygen partial pressures between 0.5 and 21,000 Pa. Diffusion mobility was measured in Ar/O₂ gas flow at pO₂ = 18 Pa. Crystal structure of this compound was found to be stable at the mentioned experimental conditions. LaNi_0.6Fe_0.4O_3−δ is a p-type semiconductor with metallic type conductivity above 150 °C at the investigated pO₂ range. Two different (fast and slow) oxygen exchange areas on the temperature-pO₂ diagram were established, which are due to two different oxygen anion positions in the double B-site mixed perovskite structure. Oxygen non-stoichiometry in the fast oxygen exchange region reaches about 0.005 of oxygen atomic index. Chemical diffusion and oxygen surface exchange coefficients do not vary at 600-800 °C, but show visible increase above 800-850 °C.     

Electrode reactions of La0.8Sr0.2MnO3±δ-electrodes on stabilized zirconia with oxygen and the nitrogen oxides NO and NO2

The kinetics for the electrode reactions with oxygen and with NO and NO₂ in the presence of oxygen has been studied for La_0.8Sr_0.2MnO_3±δ-electrodes on stabilized zirconia (8 mol% Y₂O₃=YSZ) in the temperature range between 500°C and 900°C for oxygen partial pressures between 1 kPa and 20 kPa by means of electrochemical methods (impedance, I-U characteristics) and temperature programmed desorption (TPD). For oxygen reduction below 900°C a mechanism is proposed which describes the formation of peroxidic ions at the electrode surface and a subsequent rate-determining electron transfer at the three-phase-boundary. At temperatures below 650°C the electrode reaction between NO and NO₂ is much faster than the oxygen reduction. The results for the NO₂-reduction to NO can be explained by a two-step mechanism consisting of a fast one-electron transfer to adsorbed NO₂ at the electrode surface and a subsequent rate-determining transfer of the second electron to NO₂ at the three-phase-boundary. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11 – 18 Sept. 1994     

Study of the oxygen incorporation and diffusion in Sr(Ti0.65Fe0.35)O3 ceramics

For applications in high temperature fast conductometric gas sensors and oxygen membranes, several mixed conductors show promising features. In particular, acceptor-doped strontium titanate (STO) has been widely investigated for an application as a fast conductometric oxygen sensor. By a B-site substitution with 35% iron, the resulting ceramic solid solution SrTi_0.65Fe_0.35O₃ (STF35) exhibits a temperature-independent conductivity, an ideal prerequisite for a gas sensor.In the presented study, the oxygen tracer exchange behavior and the tracer diffusion of dense ceramic STF35 bulks have been investigated in the temperature range between 600 and 900 °C by means of ¹⁸O₂ tracer exchange experiments and subsequent secondary ion mass spectrometry (SIMS), resulting in the determination of k* and D* values, respectively.Furthermore, by coating the samples with a thin alkaline earth metal oxide layer (CaO), a significantly enhanced oxygen surface exchange reaction was observed. These findings are in good agreement with previous results on STO single crystals.     

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.     

Enhancement of oxygen surface kinetics of SrTiO3 by alkaline earth metal oxides

The kinetics of oxygen incorporation into semiconducting metal oxides such as strontium titanate are of particular interest for an application as a fast resistive-type oxygen sensor operating at high temperatures (T > 600 °C), e.g. in automotive exhaust gas monitoring.Based on a frequency-domain analysis of the response signal (resistance R) obtained from an electrically contacted sample exposed to a modulated oxygen partial pressure pO₂ in a fast kinetic measurement setup, the surface transfer controlled response behaviour of undoped and Fe-doped SrTiO₃ single crystals with thicknesses in the order of d = 45–125 μm was investigated with regard to the effect of thin PVD layers with alkaline earth (Ca, Sr, Ba) metal oxide compounds.The results of this electrical characterisation are compared with results obtained from ¹⁸O tracer exchange experiments and subsequent depth profile analysis by secondary ion mass spectrometry (SIMS). Both the electrical measurements and the ¹⁸O self-diffusion experiments revealed an enhancement of oxygen surface exchange kinetics in the case of all three alkaline earth metal oxides investigated.     

Measurement of mixed conductivity in thin films with microstructured Hebb–Wagner blocking electrodes

Determination of the ionic and electronic contributions to the total conductivity in mixed ionic–electronic conductors (MIEC) is central to understanding their properties, particularly in nanostructured ionic solids. The Hebb–Wagner blocking technique, commonly used to deconvolute ionic and electronic contributions in bulk MIECs, is susceptible to misinterpretation when applied to thin films. In this work, microfabricated electronic blocking electrodes consisting of porous Pt on dense thin yttria-stabilized zirconia (YSZ) films were applied to nanocrystalline CeO₂ thin films. The validity of the blocking structure was expressly considered with respect to alternate current and gas phase reaction pathways, with criteria developed to aid in identifying spurious effects. The ionic partial conductivity in nanocrystalline CeO₂ thin films was confirmed to be pO₂-independent while the electronic partial conductivity was found to be pO₂ dependent with a power dependence of − 0.31 ± 0.02. These results are compared with theoretical predictions of extrinsically-compensated ceria and previous results on bulk nanocrystalline ceria.     

Room temperature response of SnO2-electrode modified solid state electrochemical CO2 sensors

A thin film solid state electrochemical gas sensor has been investigated for CO₂ detection based on the cell reaction: Na⁺+OH+CO₂=NaHCO₃. The galvanic cell arrangement is Au | Na_xCoO_2−δ (ref.) | NASICON | Au, SnO₂ where the right hand electrode is in contact with CO₂ and O₂ in a humid atmosphere. The response has been compared to results obtained with a conventional pellet type sensor. Furthermore, both devices have been exposed to CO and humidity. Strong cross-sensitivities were observed leading to large changes in the emf in both cases. The response to moisture is reversible and fast with a response time of about 1 min according to a fast surface reaction of H₂O with SnO₂. The presence of CO leads to a signal change with a high response time and a very slow reverse reaction. However, the response to CO₂ is not influenced by the presence of CO or H₂O with regard to the signal height and response time. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Real-time study of phase transformations in Cu-In chalcogenide thin films using in situ Raman spectroscopy and XRD

The growth of polycrystalline CuInSe₂ and CuInS₂ thin films from metallic precursor layers is investigated using two complementary in situ methods which give bulk sensitive (XRD) and surface sensitive (Raman) information. From the time evolution of the XRD and Raman peak intensities the phase transformation sequences and the reaction kinetics can be derived. In both cases the chalcogenization of the Cu-In precursors proceeds at the top surface. Thus, the process is at least limited by cation diffusion through the metallic precursor. However, the growth kinetics of CuInSe₂ and CuInS₂ films differ. While the CuInSe₂ growth is limited by the reaction of binary phases, CuInS₂ growth is controlled by fast diffusion which is solely restricted to the period of raising temperature during the process.     

Effect of oxygen gas pressure on orientations of Cu2O nuclei during the initial oxidation of Cu(100), (110) and (111)

The orientations of oxide nuclei during the oxidation of Cu(100), (110) and (111) surfaces have been examined by in situ transmission electron microscopy. Our results indicate that the epitaxial nucleation of oxide islands on these surfaces cannot be maintained for a whole range of oxygen gas pressure varying from 10^? 5 Torr to 750 Torr. The critical oxygen gas pressure, pO₂, leading to the transition from nucleating epitaxial to non-epitaxial oxide nuclei shows a dependence on the crystallographic orientations of the Cu substrates with pO₂⁽¹⁰⁰⁾ > pO₂⁽¹¹¹⁾ > pO₂⁽¹¹⁰⁾. By fitting the experimentally determined critical oxygen pressures to a kinetic model, we find that such dependence can be attributed to the effect of surface orientations of the Cu substrates on the oxygen surface adsorption and diffusion, which dominate the kinetic processes of oxide nucleation.     

Growth and characteristics of reactive pulsed laser deposited molybdenum trioxide thin films

Molybdenum trioxide thin films were prepared by reactive pulsed laser deposition on Corning 7059 glass substrates. The influence of oxygen partial pressure and deposition temperature on the structure, surface morphology and optical properties of these films was studied to understand the growth mechanism of MoO₃ thin films. The films formed at 473 K in an oxygen partial pressure of 100 mTorr exhibited predominantly a (0k0) orientation, corresponding to an orthorhombic layered structure of α-MoO₃. The evaluated optical band gap of the films was 3.24 eV. The crystallite size increased with increase of deposition temperature. The films formed at an oxygen partial pressure of pO₂=100 mTorr and at a deposition temperature greater than 700 K exhibited both (0k0) and (0kl) orientations, representing α-β mixed phases of MoO₃. The films formed at an oxygen partial pressure less than 100 mTorr were found to be sub-stoichiometric with α-β mixed phases. The investigation revealed the growth of polycrystalline and single-phase orthorhombic-layered-structure α-MoO₃ thin films with composition nearly approaching the nominal stoichiometry at moderate substrate temperatures in an oxygen partial pressure of 100 mTorr. Received: 9 April 2001 / Accepted: 6 August 2001 / Published online: 17 October 2001     

Measurement of oxygen transport kinetics in epitaxial La2NiO4+δ thin films by electrical conductivity relaxation

The oxygen transport kinetics of mixed ionic and electronic conducting La₂NiO₄ thin films made by pulsed laser deposition (PLD) were measured using the electrical conductivity relaxation (ECR) technique. Since the film thickness is ∼ 3000 Å, the oxygen transport kinetics are controlled by the surface exchange rate. The experimental data are not well described by the usual single time constant model for oxygen surface exchange, but a good fit is obtained using two independent time constants. This model implies that the La₂NiO₄ film consists of two independent regions with different exchange rates that correspond to two different film microstructures.     

Hydrogen gas-sensing properties of Pt/WO<Subscript>3</Subscript> thin film in various measurement conditions

A well-known gasochromic material is Pt particle-dispersed tungsten trioxide (Pt/WO₃). Its optical properties could make it effective as a hydrogen gas sensor. In this study, Pt nanoparticle-dispersed WO₃ thin films were prepared using the sol–gel process, and their optical and electrical properties dependent on the working environment (i.e., temperature, hydrogen gas concentration, oxygen partial pressure, etc.) were investigated. The Pt/WO₃ thin films prepared at 400 °C showed the largest change in optical transmittance and electrical conductivity when exposed to hydrogen gas compared with the films prepared at other temperatures. The optical absorbance and electrical conductivity were found to be dependent on the hydrogen and oxygen gas concentration in the atmosphere because generation and disappearance of W⁵⁺ in the thin films depend on the equilibrium reaction between injection and rejection of H⁺ into and from the thin films. In addition, the equilibrium reaction depends on the hydrogen and oxygen gas concentrations.     

Electrical conductivity and chemical diffusion in Perovskite-type proton conductors in H2-H2O gas mixtures

The electrical conductivities of SrZr_0.9Y_0.1O_3-δ (SZY10) and BaCe_0.95Y_0.05O_3-δ(BCY5) were measured as a function of hydrogen partial pressure P(H₂), oxygen partial pressure P(O₂), steam partial pressure P(H₂O) and temperature. Their relaxation processes were analyzed using the solution of Fick's diffusion equation to determine the chemical diffusion coefficients and surface reaction rate constants. There were the differences in chemical relaxation kinetics and the conductivity dependence on P(H₂O) between the both oxides. The chemical diffusion coefficients depend on temperature but are essentially independent of P(H₂), P(O₂) and P(H₂O). The ambipolar diffusion treatment can explain the temperature dependence of chemical diffusion coefficients quantitatively. The chemical diffusion coefficients of SZY10 is one or two order of magnitude smaller than those of BCY5 at low temperature. The sluggish conductivity relaxation in SZY10 was due to considerably small oxygen vacancy diffusion coefficients at low temperatures. The total conductivity depends on P(H₂O) in the case of SZY10, but not for BCY5. This different dependence on P(H₂O) is caused by the difference in the ratio between proton mobility and oxide-ion mobility.     

Oxygen permeation in bismuth-based materials. Part II: Characterisation of oxygen transfer in bismuth erbium oxide and bismuth calcium oxide ceramic

The ¹⁸O/¹⁶O Isotope Exchange Depth Profile technique was applied to BE25 ((Bi₂O₃)_0.75(Er₂O₃)_0.25) and BICAO ((Bi₂O₃)_0.73–(CaO)_0.27) dense ceramics to characterise the oxygen transfer in these materials. Pure ceramics, ceramics coated with silver on the surface and 40% silver cermets were investigated. Slow oxygen surface exchange kinetics were confirmed for the pure ceramics whilst the kinetics were improved when silver was painted on the surface. However oxygen permeation fluxes were not improved when a silver paste was painted onto the membrane surface. Permeation was also limited by electronic conduction which is likely to be more limiting than oxygen exchange at the surface of these ceramics. In contrast, fast kinetics of oxygen surface exchange were revealed for cermets.     

A novel humidity sensor using yb-doped SrCeO3 ionic conductor with a au-pd filter

A novel humidity sensor operating at T<580K using Yb-doped SrCeO₃ ionic conductor with a Au-Pd chemical filter has been developed. In this unique design, both the electrodes are exposed to the same test gas and therefore do not require separation of working and reference compartments. A thin layer of impervious Au-Pd alloy on one surface of the perovskite electrolyte allows selective access to oxygen gas, while the other surface coated with porous Pt is exposed to both H₂O and oxygen. Sensor emfs are found to be directly proportional to log pH₂O and independent of pO₂. The design aspects of the sensor, the experimental set-up, some typical results and a postulated mechanism of sensor operation are described in this paper. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Investigation of thin film all-solid-state lithium ion battery materials

All-solid-state thin film batteries are feasible by employing Al as anode and LiPON as electrolyte which are subsequently deposited by sputtering. The lithium ion conductivity of ∼ 10⁻⁶ S/cm for the thin film LiPON is in agreement with data reported for bulk material. The high voltage cathode Li₂CoMn₃O₈ could be prepared by forming the compound by the combustion method andsubsequent e-beam evaporation of this material with the addition of 20 wt.-% LiNO₃ at an oxygen partial pressure of 10⁻⁵ mbar. The thin film cells could be operated between 3 and 5 V vs. Al, LiAl. The chemical diffusion coefficient was found to be in the range from 10⁻¹³ to 10⁻¹² cm²/s at room temperature by employing the GIT-technique for the composition x of Li_2-xCoMn₃O₈ in the range from 0.1 to 1.6. Impedance studies of the complete battery system revealed a charge transfer resistance of 290 Θ, a double layer capacity of ∼ 45–70 μF for an electrode area of 6.7 cm² and a rate determining chemical diffusion coefficient in the range from 10⁻¹² to 10⁻¹¹ cm²/s. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15–21, 2002.     

Hf-sputtered indium oxide films doped with tin

Indium oxide films doped with tin (ITO-films) have been hf-sputtered from an 80 at-%In₂O₃/20 at-%SnO₂ target onto glass substrates. The sputter atmosphere contained mainly argon (10⁻²Torr) with addition of oxygen (0≦p _{O 2}≦2·10⁻²Torr). The sputtered films aren-conductors. The conductivity and density of charge carriers depend on the oxygen content of the sputter gas. They could be varied by two orders of magnitude. In air or in oxygen atmosphere the films oxidize at the surface and for a certain depth beneath the surface, thus decreasing the conductivity. The Hall mobility of the sputtered films is smaller (≈10 cm²V⁻¹ s⁻¹) than one observes at ITO films produced by CVD sparaying or other methods. The conductivity of as sputtered films approached maximum values of about 1000Ώ⁻¹cm⁻¹.     

Oxygen transport in La2NiO4 + δ: Assessment of surface limitations and multilayer membrane architectures

The steady-state oxygen permeation through dense La₂NiO_4 + δ ceramics, limited by both surface exchange and bulk ambipolar conduction, can be increased by deposition of porous layers onto the membrane surfaces. This makes it possible, in particular, to analyze the interfacial exchange kinetics by numerical modelling using experimental data on the oxygen fluxes and equilibrium relationships between the oxygen chemical potential, nonstoichiometry and total conductivity. The simulations showed that the role of exchange limitations increases on reducing oxygen pressure, and becomes critical at relatively large chemical potential gradients important for practical applications. The calculated oxygen diffusion coefficients in La₂NiO_4 + δ are in a good agreement with literature. In order to enhance membrane performance, the multilayer ceramics with different architecture combining dense and porous components were prepared via tape-casting and tested. The maximum oxygen fluxes were observed in the case when one dense layer, ~ 60 μm in thickness, is sandwiched between relatively thin (< 150 μm) porous layers. Whilst the permeability of such membranes is still affected by surface-exchange kinetics, increasing thickness of the porous supporting components leads to gas diffusion limitations.     

Electronic conductivity and chemical diffusion in n-conducting barium titanate ceramics at high temperatures

The electronic conductivity as well as the chemical diffusion coefficient of barium titanate ceramics doped with Y and Mn (donor-doped and acceptor co-doped) have been determined by application of conductivity relaxation experiments. The equilibrium values of the electronic conductivity of n-conducting BaTiO₃ have been analyzed by application of a defect chemical model involving electrons and cation vacancies as the predominant defect species at oxidizing conditions (fairly high oxygen partial pressures). The relaxation curves of the electronic conductivity yield the chemical diffusion coefficient of the bulk by employing a spherical grain model where the appropriate diffusion length is the radius of grains (average grain size). The conductivity relaxation experiments have been performed as a function of temperature ranging from 1100 to 1250 °C at oxygen partial pressures between 0.01 and 1 bar. The kinetics of the oxygen exchange process can be interpreted in terms of extremely fast diffusion of oxygen via oxygen vacancies along the grain boundaries and slow diffusion of Ti (cation)-vacancies from the grain boundaries into the grains. The Ti-vacancy diffusion coefficients were extracted from the chemical diffusion coefficients as a function of temperature. Typical values for the Ti-vacancy diffusivity are around 10^− 15 cm² s^− 1 with an activation energy of 3.9 ± 0.7 eV.