Activation Energy For Total Conductivity Research Articles

Properties of Barium Cerate Thin Films Formed Using E-Beam Deposition

This article focuses on the properties of the BaCeO3 thin films formed by electron-beam vapor deposition and investigates the formation of barium cerates on supports with different thermal expansion coefficients (Stainless Steel, Invar, Glass Sealing, and Inconel substrates) and the influence of the technological parameters on the properties of the formed thin films with an emphasis on the stability of the films. Morphology and phase composition and mechanical and electrical properties were investigated. It was found that the main factors influencing the phase composition and morphology of the films are the temperature of the support and the deposition rate. However, the mechanical properties of the films are mostly influenced by strains introduced to thin films by using different supports. Two interesting features of the electrical properties of the studied strained films were noticed: the film with the highest in-plane tensile strain showed the lowest activation energy of total conductivity, whereas the film with the lowest strain showed the highest value of total conductivity.

Structural and ionic conductivity of Cu-doped titania (Ti0.95Cu0.05O2−δ) for high temperature energy devices

The Cu-doped titania (Ti0.95Cu0.05O2-δ) is studied here as a solid-state ionic conductor for its possible application in high temperature energy devices such as an electrolyte for SOFC. The sample in the powder form was obtained by solid state method using TiO2 and copper acetate by heating up to 1200 °C for 10 h. It was characterized by XRD, FT-IR, Raman, SEM/EDS, DRS-UV-Visible, photoluminescence, BET and ac-impedance techniques. The oxide ion conductivity (σt) values obtained from ac-impedance measurements showed a linear increase with temperature from 300 − 700 °C. The σt values are similar to that of Ln-doped ceria, and the highest conductivity of 1.41 × 10−4 Scm−1 was recorded at 700°C. The activation energy for total conductivity was found to be 0.82 eV. The ionic and electronic transport numbers are 0.79 and 0.21, respectively. This study suggests the plausible use of rutile TiO2 based (low-cost and structurally stable) materials as electrolytes in SOFC.

Grain size effect on the electrical properties of nanocrystalline Gd2Zr2O7 ceramics

A detailed study on the effect of grain size of nanocrystalline Gd2Zr2O7 (GZO) ceramics on its electrical properties, particularly the ionic conductivity, is reported. The nanocrystalline GZO was prepared by chemical co-precipitation method. The GZO ceramic with different grain sizes from 5 to 55 nm was fabricated by sintering them at different temperatures varying from 800 to 1400 °C. Structural studies were performed by X-ray diffraction and Raman spectroscopy and found the cubic pyrochlore phase of GZO in all cases. Their crystallite size has been increased from 5 to 55 nm as well as the structural ordering has been increased with the increase in sintering temperature. The ac electrical impedance analysis showed a dramatic increase in ionic conductivity with increasing grain size. The ionic conductivity measured at 700 °C was 8.7 × 10−6, 12.5 × 10−6, 20.4 × 10−6 and 45.4 × 10−6 S cm−1 for respective grain sizes 5, 12, 25, and 55 nm of GZO ceramics. Increase in conductivity was correlated to the increase in grain size. The activation energy for grain conductivity was found to be 1.42 ± 0.01 eV for all GZO ceramics irrespective of their grain size. However, the activation energy of grain boundary and total conductivity had decreased with increasing grain size. The activation energy for total conductivity was obtained to be 1.5, 1.48, 1.46 and 1.42 eV for 5, 12, 25, and 55 nm GZO ceramics respectively. Further, the ac conductivity was found to obey the universal Jonscher's power law and the power exponent s was found to increase with the increase in temperature. It was also showed that the non-overlapping small polaron tunneling (NSPT) has been the best suited theoretical model to explain the conduction mechanism occurred in GZO ceramics. In addition, increase in co-operative dynamics of mobile ions with increase in grain size was found to favor the enhancement of conductivity.

Structure and conductivity in the Bi4Nb1−xYxO8.5−x oxide-ion conducting system

A study of structure and oxide-ion conductivity in the Bi4Nb1−xYxO8.5−x solid solution using X-ray and neutron powder diffraction and a.c. impedance spectroscopy is presented. Slow cooled samples are typically biphasic, exhibiting a mixture of cubic/pseudo-cubic and orthorhombic or tetragonal fluorite based phases. Single phase cubic and pseudo-cubic materials can be isolated by quenching from high temperatures. The defect structure in these quenched phases has been determined by neutron diffraction and shows an oxide-ion distribution that is compositionally dependent. This distribution is correlated with the compositional variation of the low temperature activation energy for total conductivity, whereas the activation energy for high temperature conductivity decreases with increasing value of x, reflecting the reduction in dopant-vacancy interaction with increasing yttrium content, as well as a change from a tetragonally ordered type III phase for low x-value compositions to a fully disordered δ-Bi2O3 type phase at x = 0.4. Ordering phenomena are proposed to account for the appearance of a linear intermediate temperature region with high activation energy in Arrhenius plots of conductivity at x ≥ 0.4. Conductivities in the order of 10−1 S cm−1 are achieved at 700 °C in these materials.

Properties of Yttrium-Doped Barium Zirconate (BZY)-Hematite Mixed Ionic-Electronic Conductor

Yttrium-doped barium zirconate (BZY), as an electrolyte material with perovskite structure for proton conducting fuel cells, exhibits high bulk proton conductivity in intermediate temperature, high chemical stability and excellent mechanical strength. BaZr0.8Y0.2O3-δ (BZY20) precursors were prepared by co-precipitation method and calcined at 1200 ˚C to form fine BZY powders. Then after pressing the BZY20 powder into pellets, the pellets were sintered at 1400 ˚C with 1 wt% nickel oxide as sintering aid for a period of 12 hours and 20 hours. The relative density of the sintered pellets of BZY (sintering 12 hours) and BZY (sintering 20 hours) rose from 95% to 96% (with respect to density of barium zirconate which is 6.21 g/cm3). SEM micrographs of the sintered BZY20 confirms the high density. BZY-hematite composite powders with different weight ratios of BZY powders and hematite nanoparticles (1:10, 10:10, and 10:20 in weight ratio) were prepared by ball-milling in ethanol. Thus prepared powders were pressed into pellets by cold isostatic pressing and after sintering at 1300 ˚C -1400 ˚C for 12 hours, and the XRD pattern of the BZY-hematite (10:10) composite was measured and clear BZY phases and 2 undefined peaks appeared (Fig. 1). What’s more, the size of crystallites of the BZY-hematite (10:10) composite is 41.0 nm, and the size of BZY crystallites (sintering 12 hours) is 30.8 nm by Scherrer equation. SEM micrographs of BZY-hematite (10:10) composites showed clear crystal growth striations which have not been observed in BZY. The crystal growth striations on the crystallites of BZY-hematite composites may imply hematite exists in the grain-to-grain contact areas and thus affects the surface energy and growth rate of BZY. However, no hematite, maghemite, magnetite or other iron oxides phases could be identified from the XRD pattern of BZY-hematite composites. As maghemite, magnetite and other iron oxides demonstrate low thermal stability, it may suggest that hematite exists as amorphous state or some reactions occurred between BZY and hematite during ball milling and sintering process. In addition, iron-doped barium zirconate (BZF) powders were synthesized by sol-gel combustion method and the XRD pattern (Fig. 1) showed obvious perovskite structure. It is inferred that iron was doped in the B site rather than the A site in iron-doped barium zirconate. Fig. 2 shows the total conductivity of BZY (sintering 12 h), BZY-hematite (10:1) composite, BZY-hematite (10:20) composite, hematite, BZF and BZY (sintering 20 h) under humidified nitrogen (3% H2O) with platinum electrodes. Impedance spectra of BZY-hematite composite obtained under humidified nitrogen (3% H2O) with platinum electrodes from 120 ˚C -500 ˚C showed very high conductivity relative to BZY, which implies high electronic conductivity rather than proton conductivity (for BZY-hematite 1:10, conductivity is 1.77×10-4 S/cm under 260 ˚C, which is 100 times higher than BZY; for BZY-hematite 20:10, conductivity is 1.10×10-3 S/cm under 260 ˚C, which is 1000 times higher than BZY under 260 ˚C; and for BZF, conductivity is 5.01×10-5 S/cm under 260 ˚C). Besides the conductivity of BZY-hematite composites depended on the ratio of hematite. It is almost impossible to distinguish the arc of bulk resistance and the arc of grain boundary resistance at low temperatures for BZY-hematite composite, while those distinctions are obvious for BZY below 370 ˚C. With the addition of hematite, the total conductivity activation energy becomes lower. What’s more, the electrical conductivities of BZY-hematite 1:10, 10:10 and 10:20 were measured as 4.03×10-7, 3.84×10-6 and 5.48×10-2 S/m by direct current under room temperature which reinforces the BZY-hematite acts as a conductor while BZY is an insulator and hematite is an n-type semiconductor. Furthermore, it is very interesting that BZY-hematite composites (1:10, 10:10 and 10:20) exhibit ferromagnetism at room temperature which means those BZY-hematite composites can be attracted by magnets. However, neither BZY nor hematite shows ferromagnetism under room temperature. Besides, iron-doped barium zirconate exhibits similar physical and electrical properties as BZY which means it is an insulator with no ferromagnetism. Figure 1

Electronic and ionic conductivity of Eu0.2Ce0.8O2−δ

Abstract Eu-doped cerium oxide was synthesized and its crystal structure was analyzed by X-ray diffraction. The electrical conductivity was measured by impedance spectroscopy at 350–900 °C over a wide range of controlled oxygen partial pressures, from H 2 to O 2 , using gas mixtures. In the high pO 2 region, ionic conductivity is predominant, and the activation energy for total conductivity is 0.8 ± 0.01 eV. In low pO 2 , the conductivity is found to obey a − 1/4 power law dependence on pO 2 , with an enthalpy of reduction of 4.2 ± 0.16 eV. These results demonstrate that Eu has a similar effect on electrical properties as Gd or Sm acceptor dopants in ceria, namely, that Eu shows no evidence for multivalent behavior; rather, it appears to be a fixed valent dopant in the fluorite structured ceria, in contrast to prior studies finding multivalency of Eu in perovskite oxides under similar conditions.

Electrical Properties of La10Si6O27 Electrolytes Prepared By an Innovative Chemical Route

Apatite-type lanthanum silicates have been widely used as solid oxide fuel cell (SOFC) electrolytes due to their relatively high oxide ionic conductivity (around 2×10−3 Scm−1 at 800 °C) and low activation energy for conduction. In the present study, we have focused on the preparation of apatite powders of composition La10Si6O27, synthesized by a novel combination of sol-gel and co-precipitation methods, and the correlation between microstructure and electrical properties of sintered ceramics. The herein proposed synthetic approach is advantageous in reducing energy consumption and processing time, key elements to decrease the total manufacturing cost. The as-prepared powder was calcined between 500 and 900 °C, with dwell time from 2 to 4h. Phase formation of powders was determined by X-ray diffraction. The pure apatite powder was further analyzed by scanning electron microscopy (SEM). The sintering behaviour was studied by dilatometry in the temperature range of 30-1500 °C. The effects of the sintering temperature, ranging from 1350 to 1550 °C, on the microstructure and electrical properties were investigated by SEM and impedance spectroscopy. Ionic conductivity properties were measured in the 200 – 800 °C temperature range in air atmosphere. Structure of the calcined powders, characterized by X-ray diffraction (XRD) and Rietveld refinement, indicated the formation of single – phase crystalline lanthanum silicate at 900 °C. Fig. 1 shows a representative impedance spectrum for the La10Si6O27 pellet sintered at 1400 °C and measured at 600 ºC in air. The features in this plot can be attributed to contributions of grain boundary at intermediate frequency and electrode polarization at low frequency.Fig. 2 shows the calculated total electrical conductivities, for the pellets sintered at 1400 and 1450 °C, as a function of temperature following the Arrhenius law. The lanthanum silicate electrolyte sintered at 1450 °C exhibited electrical properties comparable with samples prepared by solid state reaction and sintered at 1600 °C [1]. The findings in this study are an indication for the potential of an innovative chemical route, based on a combination between sol-gel and co-precipitation methods, for the synthesis of lanthanum silicate powders with apatite-type structure. Nanoparticles were obtained even after calcination at temperatures as high as 900 °C. Densities above 90% of the theoretical one were achieved after sintering at 1450 °C. The total electrical conductivity at 600 °C was comparable with the best values reported in the literature. The lower activation energy of total conductivity was 80 kJ/mol. Acknowledgements The authors acknowledge CAPES and FCT (Portugal) for the financial support. Reference [1] E. Béchade et al. Journal of the European Ceramic Society 28, 2717 (2008).

Studies on structural, morphological and electrical properties of Ce0.8Ln0.2O2−δ (Ln=Y3+, Gd3+, Sm3+, Nd3+ and La3+) solid solutions prepared by citrate complexation method

Abstract A modified sol–gel method which is also known as citrate complexation is used here to prepare Ce 0.8 Ln 0.2 O 2− δ (Ln = Y 3+ , Gd 3+ , Sm 3+ , Nd 3+ and La 3+ ) solid solutions for their proposed application in intermediate-temperature solid oxide fuel cells (IT-SOFCs) as electrolytes. All the samples exhibit the fluorite type crystalline structure without any phase segregation, characteristics of CeO 2 , as revealed in XRD pattern. The lattice parameters and densities calculated based on the oxygen vacancy model agreed well with the experimental (Archimedes method) values. The formation of solid solution is also confirmed by Raman spectroscopy. The microstructural features of the samples are recorded by using FE-SEM/TEM. The ionic conductivities of various Ln-doped samples at 623 K as obtained from electrochemical impedance measurement are as follows: Ce 0.8 Sm 0.2 O 2− δ > Ce 0.8 Gd 0.2 O 2− δ > Ce 0.8 Nd 0.2 O 2− δ > Ce 0.8 La 0.2 O 2− δ > Ce 0.8 Y 0.2 O 2− δ . The activation energy for total conductivity are found to be 0.86, 0.87, 0.89, 0.96 and 1.02 eV for Sm, Gd, Nd, La and Y doped ceria, respectively. The relationship between the dopant radius, chemical composition, lattice parameters, morphology and electric properties are discussed here.

Transport, thermomechanical, and electrode properties of perovskite-type $$ {\left( {{\hbox{L}}{{\hbox{a}}_{0.{75} - x}}{\hbox{S}}{{\hbox{r}}_{0.{25} + x}}} \right)_{0.{95}}}{\hbox{M}}{{\hbox{n}}_{0.{5}}}{\hbox{C}}{{\hbox{r}}_{0.{5} - x}}{\hbox{T}}{{\hbox{i}}_x}{{\hbox{O}}_{{3} - }}_\delta \left( {x = 0 - 0.{5}} \right) $$

Increasing Sr2+ and Ti4+ concentrations in perovskite-type \( {\left( {{\hbox{L}}{{\hbox{a}}_{0.{75} - x}}{\hbox{S}}{{\hbox{r}}_{0.{25} + x}}} \right)_{0.{95}}}{\hbox{M}}{{\hbox{n}}_{0.{5}}}{\hbox{C}}{{\hbox{r}}_{0.{5} - x}}{\hbox{T}}{{\hbox{i}}_x}{{\hbox{O}}_{{3} - }}_\delta \left( {x = 0 - 0.{5}} \right) \) results in slightly higher thermal and chemical expansion, whereas the total conductivity activation energy tends to decrease. The average thermal expansion coefficients determined by controlled-atmosphere dilatometry vary in the range (10.8–14.5) × 10−6 K−1 at 373–1,373 K, being almost independent of the oxygen partial pressure. Variations of the conductivity and Seebeck coefficient, studied in the oxygen pressure range 10−18–0.5 atm, suggest that the electronic transport under oxidizing and moderately reducing conditions is dominated by p-type charge carriers and occurs via a small-polaron mechanism. Contrary to the hole concentration changes, the hole mobility decreases with increasing x. The oxygen permeation fluxes through dense ceramic membranes are quite similar for all compositions due to very low level of oxygen nonstoichiometry and are strongly affected by the grain-boundary diffusion and surface exchange kinetics. The porous electrodes applied onto lanthanum gallate-based solid electrolyte exhibit a considerably better electrochemical performance compared to the apatite-type La10Si5AlO26.5 electrolyte at atmospheric oxygen pressure, while Sr2+ and Ti4+ additions have no essential influence on the polarization resistance. In H2-containing gases where the electronic transport in \( {\left( {{\hbox{L}}{{\hbox{a}}_{0.{75} - x}}{\hbox{S}}{{\hbox{r}}_{0.{25} + x}}} \right)_{0.{95}}}{\hbox{M}}{{\hbox{n}}_{0.{5}}}{\hbox{C}}{{\hbox{r}}_{0.{5} - x}}{\hbox{T}}{{\hbox{i}}_x}{{\hbox{O}}_{{3} - }}_\delta \) perovskites becomes low, co-doping deteriorates the anode performance, which can be however improved by infiltrating Ni and \( {\hbox{Ce}}{{\hbox{O}}_{{\rm{2}} - }}_\delta \)v into the porous oxide electrode matrix.

Synthesis and electrophysical properties of (1 − x)Ce 0.8Gd 0.2O 2 − δ + xTiO 2 ( x = 0–0.06) solid-state solutions

In the present work the electrical properties of (1 − x)Ce 0.8Gd 0.2O 2 − δ + xTiO 2 ( x = 0–0.06) synthesized by the standard ceramic technology were investigated, in a wide range of oxygen partial pressures (0.21–10 − 20 atm at 900 °C) and temperatures (600–900 °C), both in air and H 2 + 3%H 2O atmosphere. It was found, that only the introduction of above 1 mol% TiO 2 decreases the sintering temperature of Ce 0.8Gd 0.2O 2 − δ . The values of total electrical conductivity do not change significantly with the content of the sintering aid up to 3 mol%, while the activation energy of total conductivity in air decreases considerably with the introduction of TiO 2 into Ce 0.8Gd 0.2O 2 − δ from 87 to 67 kJ/mol; however, it is independent of x up to 0.03. The dependence of the total conductivity on the oxygen partial pressure is characterized by three regions. In the intermediate region (10 − 10 –10 − 14 ) the conductivity behavior is described according to the oxygen vacancy model. The n-type conductivity region curve of Ce 0.8Gd 0.2O 2 − δ is fitted to a -1/5 slope. The increase of TiO 2 content leads to the slope increase at − 1/6, − 1/8 and − 1/9 for 1, 2 and 3 mol% TiO 2, respectively, due to the trapping of electrons by Ti 4+. 1 mol% doping increases the electrolytic region of Ce 0.8Gd 0.2O 2 − δ from 10 − 10 to 10 − 13 atm. Finally, the optimal content of TiO 2 from the viewpoint of sintering and electrical properties was found to be 2 mol%.

The effect of cation nonstoichiometry on the electrical conductivity of acceptor-doped CaZrO 3

The electrical properties of acceptor-doped Ca 1− x Zr 0.99M 0.01O 3− δ (M = Mg 2+, In 3+) systems were investigated as a function of cation nonstoichiometry (0 ≤ x ≤ 0.05). The characterization was carried out using the impedance spectroscopy between 550 °C and 1100 °C in dry air. The contributions of the grain and grain boundary conductivity to the total conductivity were obtained from the impedance data. When the Ca deficiency ( x) increased, the total conductivity rapidly decreased with the corresponding increase in activation energy. Although the grain conductivity increased slightly with increasing x, the total conductivity is mostly determined by the highly resistive grain boundary. With varying x, the activation energy of total conductivity showed the percolation behavior. The percolation threshold values vary according to the doped species. It may be due to the difference in concentration of oxygen vacancies of the specimens.