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EMF transport number measurements

In ProboStat a disk sample can be suspended with suitable sealing material to form two separate gas chambers and each chamber supplied with desired gas mixture.

Using top and bottom (and maybe guard) electrodes the user then performs measurements with desired gradients over the sample. The voltage is measured by a high-impedance voltmeter, and is a measure of ionic transport numbers in the sample.

Measurements of total conductivity are easily performed during the emf-measurements by manual switching or automatic multiplexing of the electrodes, and the combination of these two methods is a powerful tool in the investigation of the electrical conductivity of a material.

These articles refer to ProboStat or other NorECs products, filtered with keywords: 'Transport number, Transport properties'  

Sm6-xMoO12-δ (x = 0, 0.5) and Sm6WO12 – Mixed electron-proton conducting materials

Authors A.V. Shlyakhtina, S.N. Savvin, N.V. Lyskov, D.A. Belov, A.N. Shchegolikhin, I.V. Kolbanev, O.K. Karyagina, S.A. Chernyak, L.G. Shcherbakova, P. Nez
Solid State Ionics
Time of Publication: 2017
Abstract Samarium molybdates Sm6-xMoO12-δ (x = 0, 0.5) and samarium tungstate Sm6WO12 – potential mixed electron-proton conductors have been studied by X-ray diffraction, Raman spectroscopy, SEM and impedance spectroscopy (in ambient air and in dry and wet air). Solid solutions differing in structure have been obtained in the Sm2O3-MoO3 system at 1600 C. The samarium molybdate Sm6MoO12 has the fluorite structure (Fm3m). The less samarium rich solid solution Sm5.5MoO11.25 crystallizes in a rhombohedral (View the MathML sourceR3) structure. The morphotropic transformation is due to the change in the chemical composition of the solid solution with decreasing Sm3 + concentration. The total conductivity of the cubic fluorite phase Sm6MoO12 at 750 C in air (1.48 10− 3 S/cm, Ea = 1.22 eV) is an order of magnitude higher than that of rhombohedral Sm5.5MoO11.25 (2.34 10− 4 S/cm, Ea = 1.11 eV). At low temperatures (T < 500 C), the Arrhenius plot of total conductivity for Sm6MoO12 and Sm5.5MoO11.25 in air deviates from linearity, suggesting that there is a proton contribution to its conductivity at these temperatures, like in the case of the Sm5.4Zr0.6MoO12.3 zirconium-doped molybdate. Below ~ 500 C, Sm6MoO12 fluorite and fluorite-like Sm6WO12 have identical Arrhenius plots of conductivity in ambient air. The region of dominant proton conductivity is wider for Sm6WO12 than Sm6MoO12, reaching temperatures as high as 750 С for the former. The absolute values of total conductivity obtained for samarium tungstate and molybdate at 400 С in wet air are virtually identical and close to 3 10− 6 S/cm, which suggests the conductivity of both compounds is dominated by protons at low temperatures and the proton transport numbers are similar.
Keywords Phase transition; Fluorite; Fluorite-like phase; Proton-conducting membranes; Proton conductivity; Electron conductivity

Tailoring transport properties through nonstoichiometry in BaTiO3–BiScO3 and SrTiO3–Bi(Zn1/2Ti1/2)O3 for capacitor applications

Authors Nitish Kumar, David P. Cann
Journal of Materials Science
Volume: 51, Issue: 20, Pages: 9404–9414
Time of Publication: 2016
Abstract The ceramic perovskite solid solutions BaTiO3–BiScO3 (BT–BS) and SrTiO3–Bi(Zn1/2Ti1/2)O3 (ST–BZT) are promising candidates for high-temperature and high-energy density dielectric applications. A-site cation nonstoichiometry was introduced in these two ceramic systems to investigate their effects on the dielectric and transport properties using temperature- and oxygen partial pressure-dependent AC impedance spectroscopy. For p-type BT–BS ceramics, the addition of excess Bi led to effective donor doping along with a significant improvement in insulation properties. A similar effect was observed on introducing Ba vacancies onto the A-sublattice. However, Bi deficiency registered an opposite effect with effective acceptor doping and a deterioration in the bulk resistivity values. For n-type intrinsic ST–BZT ceramics, the addition of excess Sr onto the A-sublattice resulted in a decrease in resistivity values, as expected. Introduction of Sr vacancies or addition of excess Bi on A-site did not appear to affect the insulation properties in air. These results indicate that minor levels of nonstoichiometry can have an important impact on the material properties, and furthermore it demonstrates the difficulties encountered in trying to establish a general model for the defect chemistry of Bi-containing perovskite systems.
Remark DOI: 10.1007/s10853-016-0186-z

Controlling mixed conductivity in Na1/2Bi1/2TiO3 using A-site non-stoichiometry and Nb-donor doping

Authors Linhao Li, Ming Li, Huairuo Zhang, Ian M. Reaney and Derek C. Sinclair
J. Mater. Chem. C
Volume: 4, Pages: 5779-5786
Time of Publication: 2016
Abstract Precise control of electronic and/or ionic conductivity in electroceramics is crucial to achieve the desired functional properties as well as to improve manufacturing practices. We recently reported the conventional piezoelectric material Na1/2Bi1/2TiO3 (NBT) can be tuned into a novel oxide-ion conductor with an oxide-ion transport number (tion) > 0.9 by creating bismuth and oxygen vacancies. A small Bi-excess in the nominal starting composition (Na0.50Bi0.50+xTiO3+3x/2, x = 0.01) or Nb-donor doping (Na0.50Bi0.50Ti1−yNbyO3+y/2, 0.005 ≤ y ≤ 0.030) can reduce significantly the electrical conductivity to create dielectric behaviour by filling oxygen vacancies and suppressing oxide ion conduction (tion ≤ 0.10). Here we show a further increase in the starting Bi-excess content (0.02 ≤ x ≤ 0.10) reintroduces significant levels of oxide-ion conductivity and increases tion ∼ 0.4–0.6 to create mixed ionic/electronic behaviour. The switch from insulating to mixed conducting behaviour for x > 0.01 is linked to the presence of Bi-rich secondary phases and we discuss possible explanations for this effect. Mixed conducting behaviour with tion ∼ 0.5–0.6 can also be achieved with lower levels of Nb-doping (y ∼ 0.003) due to incomplete filling of oxygen vacancies without the presence of secondary phases. NBT can now be compositionally tailored to exhibit three types of electrical behaviour; Type I (oxide-ion conductor); Type II (mixed ionic-electronic conductor); Type III (insulator) and these results reveal an approach to fine-tune tion in NBT from near unity to zero. In addition to developing new oxide-ion and now mixed ionic/electronic NBT-based conductors, this flexibility in control of oxygen vacancies allows fine-tuning of both the dielectric/piezoelectric properties and design manufacturing practices for NBT-based multilayer piezoelectric devices.
Remark DOI: 10.1039/C6TC01719C

Experimental and molecular dynamics study of thermo-physical and transport properties of ThO2-5wt.%CeO2 mixed oxides

Authors P.S. Somayajulu, P.S. Ghosh, J. Banerjee, K.L.N.C. Babu, K.M. Danny, B.P. Mandal, T. Mahata, P. Sengupta, S.K. Sali, A. Arya
Journal of Nuclear Materials
Volume: 467, Issue: 2, Pages: 644–659
Time of Publication: 2015
Abstract We have determined the thermo-physical (elastic modulus, specific heat, thermal expansion and thermal conductivity) and transport (ionic conductivity) properties of ThO2-5wt.%CeO2 mixed oxide (MOX) using a combined experimental and theoretical methodology. The specific heat, ionic conductivity and elastic properties of ThO2-5wt.%CeO2 pellets prepared by conventional powder metallurgy (POP) and coated agglomerate pelletization (CAP) routes (sintered in both air and Ar-8%H2 atmosphere) are compared with respect to homogeneity (CeO2 distribution in ThO2 matrix), microstructure, porosity and oxygen to metal ratio. The effects of inhomogeneity and pore distribution on thermal expansion and thermal conductivity of the mixed-oxide pellets are identified. Molecular dynamics (MD) simulations using the Coulomb-Buckingham-Morse-many-body model based interatomic potentials are used to predict elastic properties in the temperature range between 300 and 2000 K and thermodynamic properties, viz., enthalpy increment and specific heats of ThO2. Finally, the thermal expansion coefficient and thermal conductivity of ThO2 and (Th,Ce)O2 mixed-oxides obtained from MD are compared with available experimental results.
Keywords ThO2-5%CeO2 MOX; Specific heat; Ionic conductivity; Temperature dependent elastic properties; Molecular dynamics simulation
Remark doi:10.1016/j.jnucmat.2015.10.053

Protons in piezoelectric langatate; La3Ga5.5Ta0.5O14

Authors Tor Svendsen Bjrheim, Vijay Shanmugappirabu, Reidar Haugsrud, Truls E. Norby
Solid State Ionics
Volume: 278, Pages: 275–280
Time of Publication: 2015
Abstract This contribution reports the hydration and electrical transport properties of effectively acceptor doped single crystalline and polycrystalline langatate, La3Ga5.5Ta0.5O14. The electrical properties are investigated over wide ranges of pH2OpH2O, pD2OpD2O and pO2pO2 in the temperature range 400 to 1000 C. Acceptor doped langatate is dominated by oxygen vacancies in dry atmospheres and at high temperatures, and by protonic defects in wet atmospheres and at lower temperatures. The corresponding standard hydration enthalpy and entropy are − 90 5 kJ/mol and − 130 5 J/mol K, respectively. Further, all compositions display pure proton conductivity in wet atmospheres below 700 C with a proton mobility enthalpy in the range of 70–75 kJ/mol, depending on doping level and crystallographic direction. Hence, protons are important for the physiochemical properties of langatate even at 1000 C, and could therefore influence the behavior of langatate-based resonator devices. The proton conductivity is slightly anisotropic, being higher in the X- and Y- than in the Z-direction. At high temperatures and under dry conditions, electron holes and oxide ions dominate the conductivity, and the enthalpy of mobility of vacancies is 140 5 kJ/mol.
Keywords Langatate; Piezoelectric; Defects; Protons; Conductivity
Remark doi:10.1016/j.ssi.2015.06.024

Tetragonal tungsten bronzes Nb8−xW9+xO47−δ: optimization strategies and transport properties of a new n-type thermoelectric oxide

Authors Christophe P. Heinrich, Matthias Schrade, Giacomo Cerretti, Ingo Lieberwirth, Patrick Leidich, Andreas Schmitz, Harald Fjeld, Eckhard Mueller, Terje G. Finstad, Truls Norby and Wolfgang Tremel
Materials Horizons
Issue: 5, Pages: 519-527
Time of Publication: 2015
Abstract Engineering of nanoscaled structures may help controlling the electrical and thermal transport in solids, in particular for thermoelectric applications that require the combination of low thermal conductivity and low electrical resistivity. The tetragonal tungsten bronzes Nb8−xW9+xO47 (TTB) allow a continuous variation of the charge carrier concentration while fulfilling at the same time the concept of a “phonon-glass electron-crystal” through a layered nanostructure defined by intrinsic crystallographic shear planes. The thermoelectric properties of the tetragonal tungsten bronzes Nb8−xW9+xO47−δ (0 < x < 2) were studied in the temperature range from 373 to 973 K. Structural defects and the thermal stability under various oxygen partial pressure pO2 were investigated by means of thermogravimetry, HR-TEM, and XRD. Nb8W9O47−δ was found stable at 973 K and a pO2 of ≈10−15 atm. The oxygen nonstoichiometry δ can reach up to 0.3, depending on the applied atmosphere. By increasing the substitution level x, the electrical resistivity ρ and the Seebeck coefficient S decreased. For x = 2, ρ reached 20 mΩ cm at 973 K, combined with a Seebeck coefficient of approximately −120 μV K−1. The thermal conductivity was low for all samples, ranging from 1.6 to 2.0 W K−1 m−1, attributed to the complex crystal structure. The best thermoelectric figure of merit zT of the investigated samples was 0.043, obtained for x = 2 at 973 K, but it is expected to increase significantly upon a further increase of x. The control of the oxygen non-stoichiometry δ opens a second independent optimization strategy for tetragonal tungsten bronzes.
Remark DOI: 10.1039/C5MH00033E

Oxide ion transport in (Nd2−xZrx)Zr2O7+δ electrolytes by an interstitial mechanism

Authors A.V. Shlyakhtina, D.A. Belov, A.V. Knotko, M. Avdeev, I.V. Kolbanev, G.A. Vorobieva, O.K. Karyagina, L.G. Shcherbakova
Journal of Alloys and Compounds
Volume: 603, Issue: 5, Pages: 274–281
Time of Publication: 2014
Abstract We have studied the structure and transport properties of ten (Nd2−xZrx)Zr2O7+x/2 (x = 0–1.27) solid solutions, which lie in the ZrO2–Nd2Zr2O7 isomorphous miscibility range. Major attention has been focused on the pyrochlore-like (Nd2−xZrx)Zr2O7+x/2 solid solutions with x = 0–0.78, which are thought to be potential interstitial oxide ion conductors. The X-ray and neutron diffraction results demonstrate that the (Nd2−xZrx)Zr2O7+x/2 (x = 0–1.27) solid solutions undergo an order–disorder (pyrochlore–defect fluorite) structural phase transition. The (Nd2−xZrx)Zr2O7+x/2 (x = 0.2–0.78) have the bulk conductivity, ∼(1.2–4) 10–3 S/cm at 750 C, which is two orders of magnitude higher than that of the ordered pyrochlore Nd2Zr2O7. An attempt has been made to determine the interstitial oxygen content of (Nd2−xZrx)Zr2O7+x/2 (x = 0.2; 0.67) in a reducing atmosphere using thermogravimetry and mass spectrometry. It has been shown that no reduction occurs in the NdZrO system, where neodymium has only one oxidation state, 3+.
Keywords Fuel cells; Ionic conduction; Electrochemical impedance spectroscopy; Neutron diffraction; X-ray diffraction; SEM

Structure and transport properties in un-doped and acceptor-doped gadolinium tungstates

Authors Wen Xing, Protima Rauwel, Charles H. Hervoches, Zuoan Li, Reidar Haugsrud
Solid State Ionics
Volume: 261, Pages: 87-94
Time of Publication: 2014
Abstract Nominal Gd6WO12, Gd5.94Ca0.06WO12 − δ, Gd5.7Ca0.3WO12 − δ and Gd5.7WO12 − δ were synthesized by solid state reaction and wet chemistry methods. The structure and morphology of the materials were analyzed by XRD, SEM and TEM and the electrical conductivity was measured as a function of temperature in reducing and oxidizing atmospheres under wet and dry conditions. The total conductivity is essentially independent of composition above 700 C. Below 700 C, the conductivity of Ca-doped samples is higher than that of Gd6WO12 and Gd5.7WO12 − δ and increases with increasing doping concentration. The conductivity below 700 C is also higher under wet compared to dry conditions and, moreover, the H–D isotope effect on the conductivity is significant. Based on this, and on conductivity characterization as a function of pO2pO2 and pH2OpH2O, it was concluded that the materials are mixed ionic and electronic conductors where electrons and holes dominate at high temperatures and intermediate temperatures under sufficiently reducing and oxidizing conditions, respectively. Protons are the predominating ionic charge carriers below approximately 700 C. The hydrogen flux through Gd5.7Ca0.3WO12 − δ was measured as a function of temperature under wet and dry sweep gas conditions, as well as with varying pH2pH2 on the feed side, confirming the picture outlined by the conductivity measurements. A defect chemical model has been derived to which the conductivity data were fitted yielding thermodynamic and transport parameters describing the functional characteristics of the materials.
Keywords Proton; Structure; Gd6WO12; Ambipolar conductivity; Hydrogen flux
Remark Link

Investigation of La1−xSrxCrO3−∂ (x ~ 0.1) as Membrane for Hydrogen Production

Authors Yngve Larring, Camilla Vigen, Florian Ahouanto, Marie-Laure Fontaine, Thijs Peters, Jens B. Smith, Truls Norby and Rune Bredesen
Volume: 2, Issue: 3, Pages: 665-686
Time of Publication: 2012-09
Abstract Various inorganic membranes have demonstrated good capability to separate hydrogen from other gases at elevated temperatures. Hydrogen-permeable, dense, mixed proton-electron conducting ceramic oxides offer superior selectivity and thermal stability, but chemically robust candidates with higher ambipolar protonic and electronic conductivity are needed. In this work, we present for the first time the results of various investigations of La1−xSrxCrO3−∂ membranes for hydrogen production. We aim in particular to elucidate the material’s complex transport properties, involving co-ionic transport of oxide ions and protons, in addition to electron holes. This opens some new possibilities for efficient heat and mass transfer management in the production of hydrogen. Conductivity measurements as a function of pH2 at constant pO2 exhibit changes that reveal a significant hydration and presence of protons. The flux and production of hydrogen have been measured under different chemical gradients. In particular, the effect of water vapor in the feed and permeate gas stream sides was investigated with the aim of quantifying the ratio of hydrogen production by hydrogen flux from feed to permeate and oxygen flux the opposite way (“water splitting”). Deuterium labeling was used to unambiguously prove flux of hydrogen species.
Keywords hydrogen transport membrane; proton permeation; oxygen permeation; water splitting
Remark Link

Nd-doped Ba(Ce,Zr)O3 − δ proton conductors for application in conversion of CO2 into liquid fuels

Authors Wojciech Zając , Emil Hanc, Agnieszka Gorzkowska-Sobas, Konrad Świerczek, Janina Molenda
Solid State Ionics
Volume: 225, Pages: 297–303
Time of Publication: 2012-10
Abstract The paper presents crystal structure, transport properties, chemical stability in CO2 atmosphere and thin film membrane preparation for materials from the Ba(Ce1 − xZrx)0.9Nd0.1O2.95 (x = 0, 0.25, 0.5, 0.75, 1) group of perovskite-type structure oxides. Transformation of crystal structure from orthorhombic Pnma to orthorhombic Imma and cubic with increasing xZr was observed along with linear decrease of pseudo-cubic unit cell volume and free lattice volume. Electrical conductivity of bulk and grain boundary was determined in dry air, as well as in air humidified with H2O or D2O. The highest proton conductivity was observed for material with xZr = 0.25. Further increase of Zr content led to decrease of conductivity as high as 2 orders of magnitude. This effect was coupled with bell-shape dependence of activation energy and pre-exponential term. Such behavior was explained as superimposed effects of high proton mobility for zirconium-rich materials due to cubic symmetry and cerium-rich materials due to softness of oxygen–oxygen separation distance, along with high proton concentration for cerium-rich perovskites. The deteriorating effect of grain boundaries on total electrical conductivity was far more pronounced for Zr-rich materials than in the case of Ce-rich ones. Declining grain boundary conductivity was attributed to both increase of number of grain boundaries and decrease of inherent grain boundary conductivity for Zr-rich samples. The highest chemical stability in CO2 atmosphere was achieved for high-Zr content materials, on the contrary, for BaCe0.9Nd0.1O2.95 in CO2 atmosphere, the decomposition onset temperature was below 500 C. 2 μm thin film membrane of Ba(Ce0.75Zr0.25)0.9Nd0.1O2.95 was successfully prepared on c-plane sapphire and fused silica substrates. Film's crystal structure matched that of the bulk material. The electrical conductivity of thermally treated film obtained on c-plane sapphire in wet air was 3.7 10− 4 S cm− 1 at 600 C.
Keywords Proton conductors; BaCeO3–BaZrO3 solid solutions; Isotope effect; Grain boundary effect; Thin films
Remark Link

Hydrogen permeation, transport properties and microstructure of Ca-doped LaNbO4 and LaNb3O9 composites

Authors Wen Xing, Guttorm E. Syvertsen, Tor Grande, Zuoan Li, Reidar Haugsrud
Journal of Membrane Science
Volume: 415-416, Pages: 878–885
Time of Publication: 2012-10
Abstract Two composites consisting of the proton conducting Ca-doped LaNbO4 and electron conducting LaNb3O9 with respectively 90 and 70 vol% LaNbO4 were prepared by spark plasma sintering. The amount of hydrogen produced at the sweep side was measured as a function of temperature and pH2 gradient under wet and dry sweep gas conditions. The hydrogen flux increases with increasing temperature and feed-sidepH2. The flux is significantly higher for the 70 vol% LaNbO4 composite than the 90 vol% LaNbO4 composite. Ambipolar conductivities calculated from the flux data showed the same pH2 dependence for both composites. The electrical conductivity of the 70 vol% LaNbO4 composite was characterized as a function of temperature under wet hydrogen. The microstructure and phase distribution of the two composites are analyzed and their transport properties with different flux limiting processes are discussed. An increased hydrogen production with wet compared to dry sweep gas is concluded to reflect water splitting due to transport of oxygen from the permeate to the feed side.
Keywords Hydrogen flux; Proton conductivity; Ceramic-ceramic composite; LaNbO4; LaNb3O9; Ambipolar conductivity; Water splitting
Remark Link

Influence of Pr substitution on defects, transport, and grain boundary properties of acceptor-doped BaZrO3

Authors Anna Magras, Christian Kjlseth, Reidar Haugsrud, Truls Norby
International Journal of Hydrogen Energy
Volume: 37, Issue: 9, Pages: 7962–7969
Time of Publication: 2012-05
Abstract We report on effects of partially substituting Zr with the multivalent Pr on the conductivity characteristics of acceptor (Gd) doped BaZrO3-based materials. BaZr0.6Pr0.3Gd0.1O3−δ was sintered 96% dense at 1550 C with grains of 1–4 μm. The electrical conductivity was characterised by impedance spectroscopy and EMF transport number measurements as a function of temperature and the partial pressures of oxygen and water vapour. H2O/D2O exchanges were applied to further verify proton conduction. The material is mainly a mixed proton–electron conductor: the p-type electronic conductivity is ∼0.004 and ∼0.05 S/cm in wet O2 at 500 and 900 C, respectively, while the protonic conductivity is ∼10−4 S/cm and ∼10−3 S/cm. The material is expectedly a pure proton conductor at sufficiently low temperatures and wet conditions. The specific grain boundary conductivity is essentially equal for the material with or without Pr, but the overall resistance is significantly lower for the former. We propose that replacing Pr on the Zr site reduces the grain boundary contribution due to an increased grain size after otherwise equal sintering conditions.
Keywords BaZrO3; BaPrO3; Defects and transport; Grain boundaries; Grain boundary specific conductivity
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