P. Shuk

Oxide ion conducting solid electrolytes based on Bi2O3

The high oxide ion conductivity of solid solutions of bismuth oxide was initially discovered by Takahashi and coworkers. The bismuth oxide based compounds are much better solid electrolytes than the well-known stabilized zirconia. The only difficulty which has prevented their use in high temperature fuel cells and gas sensors up to now is their instability against reduction at low oxygen partial pressures. In this article, we review the structural properties, thermal expansion, electrical conductivity, thermodynamic stability, and surface properties of bismuth oxide and solid solutions of bismuth oxide with face centred cubic, rhombohedral, tetragonal or layer structures.

Mechanochemical-hydrothermal synthesis of carbonated apatite powders at room temperature

Crystalline carbonate- and sodium-and-carbonate-substituted hydroxyapatite (CO3HAp and NaCO3HAp) powders were prepared at room temperature via a heterogeneous reaction between Ca(OH)2/CaCO3/Na2CO3 and (NH4)2HPO4 aqueous solution using the mechanochemical hydrothermal route. X-ray diffraction, infrared spectroscopy, thermogravimetry, and chemical analysis were performed. Room temperature products were phase-pure CO3HAp and NaCO3HAp containing 0.8-12 wt% of carbonate ions in the lattice. Dynamic light scattering revealed that the median agglomerate size of the room temperature CO3HAp and NaCO3HAp powders was in the range of 0.35-1.6 microm with a specific surface area between 82 and 121 m2/g. Scanning and transmission electron microscopy confirmed that the carbonated HAp powders consisted of mostly submicron aggregates of nanosized, approximately 20 nm crystals. The synthesized carbonated apatite powders exhibit chemical compositions and crystallinities similar to those of mineral constituents of hard tissues and therefore are promising for fabrication of bone-resembling implants.

Solution synthesis of hydroxyapatite designer particulates

Abstract This paper reviews our research program for intelligent synthesis of hydroxyapatite (HAp) designer particulates by low-temperature hydrothermal and mechanochemical–hydrothermal methods. Our common starting point for hydrothermal crystallization is the generation and validation of equilibrium diagrams to derive the relationship between initial reaction conditions and desired phase assemblage(s). Experimental conditions were planned based on calculated phase boundaries in the system CaO–P2O5–NH4NO3–H2O at 25–200 °C. HAp powders were then hydrothermally synthesized in stirred autoclaves at 50–200 °C and by the mechanochemical–hydrothermal method in a multi-ring media mill at room temperature. The synthesized powders were characterized using X-ray diffraction, infrared spectroscopy, thermogravimetry, chemical analysis and electron microscopy. Hydrothermally synthesized HAp particle morphologies and sizes were controlled through thermodynamic and non-thermodynamic processing variables, e.g. synthesis temperature, additives and stirring speed. Hydrothermal synthesis yielded well-crystallized needle-like HAp powders (size range 20–300 nm) with minimal levels of aggregation. Conversely, room-temperature mechanochemical–hydrothermal synthesis resulted in agglomerated, nanosized (∼20 nm), mostly equiaxed particles regardless of whether the HAp was stoichiometric, carbonate-substituted, or contained both sodium and carbonate. The thermodynamic model appears to be applicable for both stoichiometric and nonstoichiometric compositions. The mechanochemical–hydrothermal technique was particularly well suited for controlling carbonate substitution in HAp powders in the range of 0.8–12 wt.%. The use of organic surfactants, pH or nonaqueous solvents facilitated the preparation of stable colloidal dispersions of these mechanochemical–hydrothermal-derived HAp nanopowders.

Properties of sol-gel prepared Ce1−xSmxO2−x/2 solid electrolytes

The structure, thermal expansion and ionic conductivity of solid electrolytes based on samarium doped cerium oxide, Ce1−xSmxO2−x2 (x = 0−0.30), prepared by the sol-gel method were systematically investigated in a wide range of temperature of 200–650 °C. The uniformly small particle size of the sol-gel prepared materials allows sintering of the samples into highly dense ceramic pellets at significantly lower temperature 1400 °C compared to that of 1600 °C required for samples prepared by solid state techniques. The ionic conductivity increases with increasing samarium substitution and reaches a maximum for the composition Ce0.80Sm0.20O1.9 (~5 × 10−3 S cm−1 at 600 °C). Thermal expansion coefficients range between 8–10 × 10−6 K−1.

Hydrothermal synthesis and properties of mixed conductors based on Ce1−xPrxO2−δ solid solutions

Abstract The structure, thermal expansion coefficients and ionic and electronic conductivities of Ce1−xPrxO2−δ (x=0–0.50) solid solutions, prepared hydrothermally for the first time, were investigated. The uniformly small particle size (35–50 nm) of the hydrothermally prepared materials allows sintering of the samples into highly dense ceramic pellets at 1300°C, a significantly lower temperature, compared to that at 1600–1650°C required for samples prepared by solid-state techniques. The maximum of total conductivity was found at x=0.30 (σ700°C=1.4×10−2 S/cm, σ600°C=5.9×10−3 S/cm; Ea=0.47 eV) with electronic contribution to the total conductivity around 50%. The thermal expansion coefficients, determined from high-temperature X-ray data, are 11.8×10−6 K−1 for CeO2 and slowly increase with increasing Pr substitution (12.4×10−6 K− at x=0.30).

Hydrothermal synthesis and properties of Ce1−xLaxO2−δ solid solutions

Abstract The structure, thermal expansion coefficients, ionic and electronic conductivities of Ce1−xLaxO2−δ (x=0–0.30) solid solutions, prepared for the first time hydrothermally, were investigated. The uniformly small particle size (25–50 nm) of the hydrothermally prepared materials allows sintering of the samples into highly dense ceramic pellets at 1300–1400°C, a significantly lower temperature, compared to that at 1600–1650°C required for ceria solid electrolytes prepared by solid state techniques. The solubility limit of La2O3 in CeO, was determined to be around 20 mol.%. The maximum conductivity, δ600°C ∼6.4×10−3 S/cm with Ea=0.73 eV, was found at x=0.15. The thermal expansion coefficients, determined from high-temperature X-ray data are 11.7±0.6×10−6 K−1 for the CeO2, and, increase with increasing La substitution (15.0×10−6 K−1 at x=0.20).

Mechanochemical-hydrothermal preparation of crystalline hydroxyapatite powders at room temperature

Crystalline hydroxyapatite (HAp) powders were prepared at room temperature from heterogeneous reaction between Ca(OH) 2 powders and (NH 4 ) 2 HPO 4 solutions via the mechanochemical-hydrothermal route. X-ray diffraction, infrared spectroscopy, thermogravimetric characterization, and chemical analysis were performed, and it was determined that the room temperature products were phase-pure, thermally stable HAp with a nearly stoichiometric composition. Dynamic light scattering revealed that the dispersed particle size distribution of the room temperature HAp powders was in the range of 0.15–3.0 μm with a specific surface area of ≈90 m 2 /g. Both specific surface area measurements and scanning electron microscopy confirmed that the HAp powders consisted of agglomerates containing hundreds of ≈20 nm HAp crystals.

Hydrothermal synthesis and properties of terbium- or praseodymium-doped Ce1−xSmxO2−x/2 solid solutions

Abstract The structure, thermal expansion coefficients and ionic conductivity of (Ce 0.83 Sm 0.17 ) 1− x (Tb/Pr) x O 1.915+δ ( x =0–0.10) solid electrolytes prepared hydrothermally were investigated. The uniformly small particle size (7–14 nm) of the hydrothermally prepared materials allows sintering of the samples into highly dense ceramic pellets at 1400°C, a significantly lower temperature, compared to that at 1600–1650°C required for samples prepared by solid state techniques. The maximum conductivity was found at x =0.10 for the Pr- and Tb-substituted ceria (σ 600°C =7.6×10 −3 S/cm, E a =0.55 eV and σ 600°C =10 −2 S/cm, E a =0.72 eV, respectively) with electronic contribution to the total conductivity around 20–30%. When the Tb or Pr substitution in Ce 0.83 Sm 0.17 O 1.915 is reduced, the conductivity becomes more ionic, and is purely ionic at 2% of Tb or Pr. However the conductivity at this lower level of doping is not significantly lower (e.g. σ 600°C =5.6×10 −3 S/cm for Tb). The thermal expansion coefficients, determined from high-temperature X-ray data, are 8.6×10 −6 K −1 for the Ce 0.83 Sm 0.17 O 1.915 and slowly increase with increasing Pr substitution and decrease with increasing Tb substitution.

Equilibration and Dynamic Phase Transitions of a Driven Vortex Lattice

We report on the observation of two types of current driven transitions in metastable vortex lattices. The metastable states, which are missed in usual slow transport measurements, are detected with a fast transport technique in the vortex lattice of undoped 2H-NbSe2. The transitions are seen by following the evolution of these states when driven by a current. At low currents we observe an equilibration transition from a metastable to a stable state, followed by a dynamic crystallization transition at high currents.

New metal-oxide-type pH sensors

Abstract Two new types of metal-oxide pH sensors with a direct solid-state contact have been developed and tested. Their performance was demonstrated near ambient temperatures with a samarium substituted ceria ceramic membrane and single crystals of several molybdenum oxide bronzes. The pH sensors with Na-molybdenum-oxide bronzes show near-ideal Nernstian behaviour in the pH range 3–9. The response was not affected by the direction of the pH change. In contrast, the pH sensor with the Na-tungsten bronze sensitive electrode (SE) shows a deviation from linearity, as well as non-Nernstian behaviour with a slope − 76.3 mV/pH at 25 °C. The response time of most molybdenum bronze pH sensors is less than 5 s for 90% response. The pH sensors with stabilized ceria membrane show close to ambient temperature non-Nernstian behaviour in the pH range 3–10 with a slope − 23.8 mV/pH at 25 °C. With increasing temperatures the behaviour of ceria membrane pH sensors approaches more and more a Nernstian response with slope − 39.9 mV/pH at 60 °C. Both pH sensors also can be used to follow acid-base titrations.