High-temperature Electrical Conductivity Measurements Research Articles

Exploring the impact of multivalent substitution on high-temperature electrical conductivity in doped lanthanum chromite perovskites: An experimental and ab-initio study

Doped LaCrO3 perovskites hold promise as robust materials for electrical interconnects and sensor applications in harsh environments. In this work, we investigated the high-temperature behavior of La1–xSrxCrO3, La1–xCaxCrO3 and La0.8Sr0.2Cr1-x MnxO3 (0.1 ≤ x ≤ 0.4) through a combination of computational modeling and physical characterization up to 1500 °C. Crystalline structural properties were determined and compared with ab-initio calculations, which demonstrated excellent agreement with experimental findings. High-temperature electrical conductivity measurements were performed under different atmospheres. Calcium and strontium/manganese co-doped lanthanum chromites exhibited typical semiconductor exponential trends and conductivity showed proportional correlation with substitutions levels up to 30%. The DFT modelling was completed up to 1500 °C, including low and high temperature chromite phases and oxygen vacancies insertion. Calculations were correlated with experimental electrical properties. This work expands the understanding of doped lanthanum chromites and paves the way for the development of materials suitable for demanding high-temperature applications.

Site preference of Y and Mn in nonstoichiometric BaTiO3 from first principles

$\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ is often doped with Y and/or Mn to realize a range of desired properties. Yet, existing canonical models of their defect chemistry cannot explain various observed phenomena outside of the high temperature electrical conductivity measurements to which they were fit. Existing models assume Y substitutes exclusively for Ba or Ti despite experiments showing Y is amphoteric, substituting predominantly for Ba or Ti depending on the cation nonstoichiometry. Existing models assume Mn forms isolated ${\mathrm{Mn}}_{\text{Ti}}$ exclusively, but experiments have shown complexes of ${\mathrm{Mn}}_{\mathrm{Ti}}$ with native oxygen vacancies (${\mathrm{Mn}}_{\mathrm{Ti}}\text{\ensuremath{-}}{\mathrm{v}}_{\text{O}}$) form in significant concentrations. Additionally, recent computational works in ${\mathrm{SrTiO}}_{3}$ suggest A-site substitutional defects may form in greater concentrations in a configuration with reduced symmetry relative to the on-site geometry. To address these inconsistencies, we developed a hybrid functional density functional theory informed grand canonical defect model with the ability to simulate specific nonstoichiometries without ad hoc assumptions about the bulk chemical potentials. Using this model, the site preference of Y and Mn in $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ as a function of the Ba/Ti ratio was evaluated in the context of a more complete set of defects including: native cation vacancies, on-site and reduced symmetry A-site defects, isolated B-site defects, and ${\mathrm{X}}_{\text{B}}\text{\ensuremath{-}}{\mathrm{v}}_{\text{O}}$ defects. The results reproduce experimental observations of yttrium's amphotericity and significant concentrations of ${\mathrm{Mn}}_{\mathrm{Ti}}\text{\ensuremath{-}}{\mathrm{v}}_{\text{O}}$. Both and Y and Mn are found to substitute predominantly for Ba at Ba/Ti = 0.99 and Ti at Ba/Ti = 1.01, but neither are found to substitute exclusively for Ba or Ti within the range of experimentally accessible Ba/Ti ratios at 1400 ${}^{\ensuremath{\circ}}\mathrm{C}$.

Structural Study and Electrical Properties of Bi1.5–xCaxSb1.5CuO7 Pyrochlore-Type Solid Solution Series

Abstract Pyrochlore solid solution Bi1.5−xCaxSb1.5CuO7 has been prepared by conventional ceramic solid state reaction with 0 ≤ x ≤ 0.4. The samples were characterized by X-ray powder diffraction (XRPD), scanning electron microscopy, and electrical conductivity measurements. The pyrochlore single phase formation was confirmed for all of the x-fractions. The variation of the electrical resistivity versus x of all samples measured by means of a four-probe device at room temperature indicates that these still remain insulators. High-temperature electrical conductivity measurements at various frequencies using a device with two electrodes allow us to note the maximum electrical conductivity of 4.7 × 10−3 S.cm−1, which has been reached at x = 0.2, generating an activation energy of 0.14 eV. For this fraction, Rietveld refinement of the corresponding XRPD pattern confirmed the typical cubic Fd-3m space group with a cell parameter of a = 10.4089(1) Å.

High-temperature properties of (La,Ca)(Fe,Mg,Mo)O3-δ perovskites as prospective electrode materials for symmetrical SOFC

La1−yCayFe0.5+x(Mg,Mo)0.5−xO3-δ oxides with the orthorhombic GdFeO3-type perovskite structure have been synthesized at 1573K. Transmission electron microscopy study for selected samples shows the coexistence of domains of perovskite phases with ordered and disordered B-cations. Mössbauer spectroscopy studies performed at 300K and 573K show that while compositions with low Ca-content (La0.55Ca0.45Fe0.5Mg0.2625Mo0.2375O3-δ and La0.5Ca0.5Fe0.6Mg0.175Mo0.225O3-δ) are nearly oxygen stoichiometric, La0.2Ca0.8Fe0.5Mg0.2625Mo0.2375O3-δ is oxygen deficient with δ ≈ 0.15. Oxides are stable in reducing atmosphere (Ar/H2, 8%) at 1173K for 12h. No additional phases have been observed at XRPD patterns of all studied perovskites and Ce1−xGdxO2−x/2 electrolyte mixtures treated at 1173–1373K, while Fe-rich compositions (x≥0.1) react with Zr1−xYxO2−x/2 electrolyte above 1273K. Dilatometry studies reveal that all samples show rather low thermal expansion coefficients (TECs) in air of 11.4–12.7ppm K−1. In reducing atmosphere their TECs were found to increase up to 12.1–15.4ppm K−1 due to chemical expansion effect. High-temperature electrical conductivity measurements in air and Ar/H2 atmosphere show that the highest conductivity is observed for Fe- and Ca-rich compositions. Moderate values of electrical conductivity and TEC together with stability towards chemical interaction with typical SOFC electrolytes make novel Fe-containing perovskites promising electrode materials for symmetrical solid oxide fuel cell.

Manufacture of Solid Oxide Fuel Cells Using Electroless Co-Deposition: A Review of Work Carried out at Edinburgh Napier University

This paper reviews work carried out at Edinburgh Napier University on the manufacture of anodes and cathodes for solid oxide fuel cells (SOFCs) using electroless co-deposition of nickel and ceramic powders. Most of the work has been focused on anodes where the co-deposition of nickel and yttria stabilised zirconia (YSZ) negates the need for traditional manufacturing methods such as screen printing and removes the need for expensive and time consuming high temperature consolidation of the ceramic part of the composite. Work has also been carried out manufacturing cathodes by electroless co-deposition replacing YSZ with lanthanum strontium manganite and lanthanum nickel ferrite. Anodes and cathodes of both planar and tubular designs of SOFCs have been successfully manufactured using this technique and process variables have included ceramic particle size, bath pH, ceramic loading, the inclusion of pore-formers and tubular orientation in the electroless bath. Testing of the electrodes included impedance analysis, long term degradation testing at 850oC for 500 and 1000-hours, high temperature electrical conductivity measurements, scanning electron microscopy and energy dispersive X-ray analysis. The cost benefits of the electroless co-deposition process compared with other conventional processes are also discussed.

Manufacture of Solid Oxide Fuel Cells Using Electroless Co-Deposition: A Review of Work Carried out at Edinburgh Napier University

This paper reviews work carried out at Edinburgh Napier University on the manufacture of anodes and cathodes for solid oxide fuel cells (SOFCs) using electroless co-deposition of nickel and ceramic powders. Most of the work has been focused on anodes where the co-deposition of nickel and yttria stabilised zirconia (YSZ) negates the need for traditional manufacturing methods such as screen printing and removes the need for expensive and time consuming high temperature consolidation of the ceramic part of the composite. Work has also been carried out manufacturing cathodes by electroless co-deposition replacing YSZ with lanthanum strontium manganite and lanthanum nickel ferrite. Anodes and cathodes of both planar and tubular designs of SOFCs have been successfully manufactured using this technique and process variables have included ceramic particle size, bath pH, ceramic loading, the inclusion of pore-formers and tubular orientation in the electroless bath. Testing of the electrodes included impedance analysis, long term degradation testing at 850oC for 500 and 1000-hours, high temperature electrical conductivity measurements, scanning electron microscopy and energy dispersive X-ray analysis. The cost benefits of the electroless co-deposition process compared with other conventional processes are also discussed.

Thermal expansion behavior and high-temperature electrical conductivity of A2−xAx′Cu1−yCoyO4±δ (A = La, Pr; A′ = Pr, Sr) oxides with the K2NiF4-type structure

Three different compositional series of Pr-containing cuprates with the K2NiF4-type structure have been prepared by annealing at 1273K, 48–90h in air: with Cu on B-site only in formal oxidation state +2 (La2−xPrxCuO4, x=0.1; 0.2; 0.4) (i), with partial replacement of Cu2+ by small (La2−xPrxCu1−yCoyO4±δ, 0.1⩽x⩽0.5; y=0.05; 0.1) (ii) and large (Pr2−xSrxCu1−xCoxO4±δ, x=0.5; 0.75) (iii) amount of cobalt. Partial replacement of Cu by Co, as in La2−xPrxCu1−yCoyO4±δ, leads to an increase of thermal expansion coefficients (TECs) in comparison with La2CuO4. Unusual high anisotropy of TEC along the a-(TEC(a)) and c-axes (TEC(c)) of the tetragonal unit cell is observed for Pr2−xSrxCu1−xCoxO4±δ, x=0.5 and 0.75. TEC(c)/TEC(a) ratio increases with increasing amount of cobalt from 3.29 (x=0.5) to 3.61 (x=0.75). High-temperature electrical conductivity measurements at 298–1173K in air reveal semiconductor-like behavior for all studied compositions with electrical conductivity values at 1173K in air vary from 1.8S/cm for La1.9Pr0.1Cu0.9Co0.1O4±δ to 5S/cm for La1.6Pr0.4CuO4±δ and to 25S/cm for Pr2−xSrxCu1−xCoxO4±δ, x=0.5, 0.75.

Synthesis, thermal expansion and high-temperature electrical conductivity of Co-doped (Y,Са)FeO3−δ with orthorhombic perovskite structure

Orthorhombic perovskites Y1−xСаxFe1−yCoyO3−δ (0.1≤х≤0.2, 0.1≤y≤0.2 and х=0.1, y=0.3) were synthesized in air by the citrate route at 1150–1300°C. High-temperature X-ray powder diffraction (HT XRPD) data for Y0.9Са0.1Fe0.8Co0.2O3−δ at 25–800°C showed no phase transition with calculated thermal expansion coefficient (TEC) of 11.9ppmK−1. High-temperature electrical conductivity measurements revealed almost composition independent conductivity values of 22–27S/cm at 900°C. No chemical interaction of Y0.8Са0.2Fe0.9Co0.1O3−δ with (Zr,Y)O2−x (YSZ) or (Ce,Gd)O2−x (GDC) was observed up to 1000°C and 1100°С, respectively. Partial replacement of Y by Pr according to formula Y0.8−zPrzCa0.2Fe0.7Co0.3O3−δ, 0.1≤z≤0.35, leads to an increase of both electrical conductivity up to 50S/cm (z=0.3) at 900°C and dilatometry measured TEC up to 15.1ppmK−1. Moderate values of electrical conductivity in combination with low TEC and stability towards chemical interaction with typical SOFC electrolytes make Co-doped Y1−xCaxFeO3−δ promissing cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFC).

High temperature electrical conductivity in undoped ceramic ZnO

The results of high temperature electrical conductivity (HTEC) measurements in undoped ceramic ZnO sample are presented. The ceramic sample under investigation was formed at high pressure and at high temperature in inert atmosphere. HTEC measurements data were obtained under the defined Zn component vapor pressure (up to 1 atm) and in temperature range (up to 1373 K). In these experiments the absolute value of HTEC in ZnO samples was several orders of magnitude higher than in undoped ZnS measured under the same conditions. Slow chemical diffusion was observed in ZnO. Slopes of HTEC isotherms in ZnO samples varied with component vapor pressure in the range from 0 to 0.2. Defect model for explanation of this experiment is discussed.

High‐Temperature Electrical Conductivity Measurements on Nanostructured Yttria‐Doped Ceria Thin Films in Ozone

We have investigated how ozone affects high‐temperature electrical properties of nanostructured yttria‐doped ceria (YDC) thin films using impedance spectroscopy. It is found that predominantly ionic conductivity in the doped‐ceria lattice is not altered upon exposure to ozone. The grain boundaries in YDC ultra‐thin films, in contrast, exhibited a substantial increase in resistance in ozone‐rich environment. While activation energy for lattice conduction in ozone was similar to air, being close to 1 eV, grain boundary activation energy increased notably from ~1.3 eV in air to 1.7 eV in ozone. Also, we have investigated kinetics of resistance change in such thin films in air and in ozone ambient using galvanostatic measurements and found that the rapid resistance change observed in ozone was reversible. Mechanisms leading to the observation and implications for ozone‐ceria film surface interactions are discussed.

INFLUENCE OF DEFECT STRUCTURE ON CATALYTIC ACTIVITY OF NANOMETRIC MATERIALS BASED ON CERIA-DOPED COPPER

A series of nanometric materials based on copper-doped cerium (IV) oxide was synthesized using a reverse (w/o) microemulsion method. The obtained powder precursors were tested by the thermal analysis methods coupled with evolved gas analysis (EGA-TGA/DTG/SDTA). Morphology and structure of the samples were characterized by XRD and BET analysis. The content of copper ions was determined using a ICP-OES spectrometer. The electronic structure of the catalysts was characterized by high-temperature electrical conductivity measurements at different pO 2. The obtained materials were tested in catalytic incineration of the selected VOCs (methanol and ethylene). The correlation between the activity of the catalysts and copper content as well as electrical properties has been confirmed.

Oxygen-ion and electron conductivity in Sr 2(Fe 1−xGa x) 2O 5

High-temperature electrical conductivity measurements, structural data from powder X-ray diffraction and 57Fe Mössbauer spectroscopy were combined to study the interrelationship of oxygen ion transport and p- and n-type transport in Sr 2(Fe 1− x Ga x ) 2O 5, where x = 0 , 0.1 and 0.2. Although gallium substitution generally decreases the total ion-electron transport, the transition of the orthorhombic brownmillerite structure to a cubic phase on heating results in the recurrence of the conductivity to the same high level as in the parent ferrite ( x = 0 ). The changes of the partial contributions to the total conductivity as a function of x are shown to reflect a complicated interplay of the disordering processes that develop in the oxygen sublattice on heating in response to replacement of iron with gallium.

High temperature electrical conductivity of undoped ZnS

High temperature defect equilibrium (HTDE) in ZnS was analysed to study the conditions of making p-type material. High temperature electrical conductivity (HTEC) under defined component vapour pressure was investigated in undoped ZnS in the temperature range from 600°C to 1140°C. HTEC measurements were carried out using a two-zone resistance furnace and a vacuum sealed quartz ampoule with four sealed tungsten or graphite electrodes. All ZnS HTEC isotherms can be divided into three regions. The high zinc vapour pressure ( p Zn) and the high sulphur vapour pressure ( p S 2 ) regions are characterized by dominant native donors. In the ZnS HTDE model, antistructure disorder is dominating at high p S 2 values, similar to CdS and CdTe HTDE models. Changes occur at low p Zn or at low p S 2 values. The increase of HTEC at low p Zn or at low p S 2 values is explained by the existence of the p-type area in this region. This p-type area appears in the total range of temperature investigated.

High temperature electrical conductivity measurements of natural diamond and diamond films

The electrical conductivity of diamond films and diamond-like carbon films were measured from room temperature to 1200°C. These results are the first known measurements for diamond films in this temperature range. The measurements were compared with the conductivity of two natural type IIa diamonds measured with the same apparatus. The activation energy for the two natural type IIa diamonds was found to be 1.4 eV, in agreement with earlier measurements on type I diamonds. The room temperature conductivities of the two diamond films were found to be higher than that of natural IIa diamond, but were found to approach or even just fall below that of natural IIa diamond at elevated temperatures. This difference, and the difference in slope of the conductivities of the various samples, are indicative of the difference in type and number of impurity and structural defects accounting for conductivity in the different samples. These results offer promise that diamond films could be fabricated which have an electrical resistivity exceeding that of natural IIa diamond.

Chemical Diffusion in Cadmium Telluride

The chemical diffusion coefficients of single crystals of cadmium telluride DCdTe were determined as a function of temperature and composition using high-temperature electrical conductivity measurements. In the temperature range 550° to 800°C, DCdTe can be approximated by 4 exp[− (1.15±0.1) eV/kT] and is nearly independent of cadmium partial pressure pCd, for pCd extending over two decades from the cadmium-rich liquidus. The measured and calculated values of DCdTe are in good agreement, and the results favor an interstitial mechanism for cadmium self-diffusion.