Harald Fjeld

Proton mobility through a second order phase transition: theoretical and experimental study of LaNbO4

The gradual change in the crystal structure of the high temperature proton conductor LaNbO(4) through a second order phase transition and its relation to the activation enthalpy of mobility of protons have been studied by means of first principles calculations and conductivity measurements. The computations have revealed that protons diffuse by an inter-tetrahedral mechanism where the activation enthalpies of mobility are 39 and 60 kJ mol(-1) in tetragonal and monoclinic LaNbO(4), respectively. The activation enthalpy of mobility of protons for tetragonal LaNbO(4), determined from the conductivity curve, is 35 kJ mol(-1). Below the transition temperature the conductivity curve bends; initially dropping off steeply, followed by a less steep decrease towards lower temperatures. The bend in the conductivity curve at the onset of the phase transition in LaNbO(4) should not be given the traditional interpretation as an abrupt change in the activation enthalpy of mobility. After application of the proper analysis of the conductivity data, which takes the second order transition into account, the activation enthalpy of mobility of protons is found to continuously increase with increasing monoclinic angle at decreasing temperature, reaching approximately 57 kJ mol(-1) at 205 degrees C for the end monoclinic phase.

Mixed Ionic and Electronic Conductivity of Undoped and Acceptor-Doped Er6WO12

The ac conductivities of Er 6 WO 12 and 1% Ca-doped Er 6 WO 12 have been characterized as a function of oxygen and water vapor partial pressures in the temperature range 300-1200°C. Partial conductivities have been determined from the open-circuit voltage of p(O 2 ), p(H 2 ), and p(H 2 O) concentration cells. The materials exhibit n- and p-type electronic conductivity and oxygen ion and proton conductivity, depending on the conditions. Protons are the major ionic charge carriers in wet atmospheres below 1150°C. Hydration thermodynamics and transport parameters have been determined under conditions where the acceptor doping is predominantly compensated by oxygen vacancies and protons.

Theoretical analysis of oxygen vacancies in layered sodium cobaltate, NaxCoO2−δ

Sodium cobaltate with high Na content is a promising thermoelectric material. It has recently been reported that oxygen vacancies can alter the material properties, reducing its figure of merit. However, experimental data concerning the oxygen stoichiometry are contradictory. We therefore studied the formation of oxygen vacancies in Na(x)CoO(2) with first principles calculations, focusing on x = 0.75. We show that a very low oxygen vacancy concentration is expected at the temperatures and partial pressures relevant for applications.

In situ studies of structural stability and proton conductivity of titanate nanotubes

Titanate nanotubes were prepared by hydrothermal treatment of anatase TiO2 with concentrated NaOH solution. In situsynchrotron X-ray diffraction studies revealed that the nanotubes are thermally unstable at temperatures above 360 °C and can transform directly to anatase via a dehydration and recrystallization process. This indicated that the titanate nanotubes possess an orthorhombic lepidocrocite (γ-FeOOH)-type layered structure. An aggregate of the as-prepared nanotubes showed an electric conductivity of about 1 × 10−6 S cm−1 at 50 °C in humid atmospheres. The conductivity depended on humidity of the atmosphere and decreased with increasing temperatures, suggesting that the proton conduction in the titanate nanotubes might correlate with the interlayer H3O+ ions.

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

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.