Search Results - (Author, Cooperation:J. B. Wagner)

2016-02-16

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

1573-4889

Springer Online Journal Archives 1860-2000

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Electronic Resource

1573-4889

Springer Online Journal Archives 1860-2000

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Electronic Resource

1573-4889

redox kinetics ; chemical diffusion ; NiO-Cr2O3 solid solutions

Springer Online Journal Archives 1860-2000

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Abstract Electrical conductivity has been measured to monitor the reequilibration kinetics for single crystals of NiO-Cr2O3 solid solutions. It has been found that the rate for the reduction process is higher than that for the oxidation runs, thus indicating that the obtained kinetic data are not purely bulk controlled. The following expressions for the apparent chemical diffusion coefficient have been obtained within the temperature range 900–1200°C and oxygen partial pressure range 1–10−5 atm: $$\begin{gathered} \tilde D_{1 red} = 1.22 \times 10^{ - 2} exp \left( {\frac{{24,420 \pm 1210 cal/mole \cdot ^\circ K}}{{RT}}} \right) \hfill \\ \tilde D_{1 oxid}^* = 1.44 \times 10^{ - 2} exp \left( {\frac{{27,340 \pm 700 cal/mole \cdot ^\circ K}}{{RT}}} \right) \hfill \\ \tilde D_{2 red} = 2.29 \times 10^{ - 2} exp \left( {\frac{{25,340 \pm 2230 calmole \cdot ^\circ K}}{{RT}}} \right) \hfill \\ \tilde D_{2 oxid}^* = 0.109 exp \left( {\frac{{29,610 \pm 3200 cal/mole \cdot ^\circ K}}{{RT}}} \right) \hfill \\ \tilde D_{3 red} = 3.16 \times 10^{ - 2} exp \left( {\frac{{26,020 \pm 2430 cal/mole \cdot ^\circ K}}{{RT}}} \right) \hfill \\ \tilde D_{3 oxid}^* = 0.202 exp \left( {\frac{{31,500 \pm 2640 cal/mole \cdot ^\circ K}}{{RT}}} \right) \hfill \\ \end{gathered} $$ .

Electronic Resource

1573-4889

chemical diffusion ; redox kinetics ; surface segregation

Springer Online Journal Archives 1860-2000

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Abstract Reequilibration kinetic data have been obtained for a model system consisting of an undoped NiO crystal covered with a thin layer of NiCr2O4. The data lead to following apparent chemical diffusion coefficients $$\tilde D^* $$ measured within the temperature range 900–1200°C when varying oxygen partial pressure from 1 to 10−5 atm: $$\begin{gathered} \tilde D^* _{red} = 5.28 \times 10^{ - 4} exp\left( {\frac{{19,160 \pm 870}}{{RT}}} \right); \hfill \\ \tilde D^* _{oxid} = 1.82 \times 10^{ - 2} exp\left( {\frac{{28,270 \pm 2,020}}{{RT}}} \right) \hfill \\ \end{gathered} $$ . Values of the apparent activation energies for the reequilibration processes were independent of the thickness of NiCr2O4 surface layer from about 0.01 to 0.05 mm. As determined by microprobe, the thickness of the NiCr2O4 surface layer changed only slightly during long reequilibration experiments. The change is in agreement with diffusion coefficients of Cr in NiO. Oxygen pressure dependence of electrical conductivity leads to the parameter 1/n = 1/5.55 being the same as those for undoped NiO crystals. This indicates the values of electrical conductivity taken at equilibrium are determined by the equilibrated undoped NiO crystal while the dissimilar reequilibration kinetic data result from the presence of the NiCr2O4 surface layer. The results indicate the slower oxidation kinetics to be surface-controlled and faster reduction kinetics to be bulk-diffusion-controlled.

Electronic Resource

1573-4889

Nickel sulfide ; activity ; chemical diffusion ; electrical conductivity ; thermodynamics

Springer Online Journal Archives 1860-2000

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Abstract The sulfur activity-composition relation of β-Ni3±xS2 was determined between 600 and 750°C by thermogravimetry using H2S/H2 gas mixtures. The existence of β1- and β2-Ni3±xS2 was confirmed. The chemical diffusion coefficient of β-Ni3±xS2 was measured as a function of composition at 650°, 700°, and 750°C using thermogravimetry for the reequilibration reaction. The chemical diffusion coefficients varied with composition and showed a maximum at stoichiometric composition, Ni3S2. The activation energy for chemical diffusion was determined as Ea=31.3 Kcal·mole−1 for stoichiometric Ni3S2. Electrical conductivity of β-Ni3+-xS2 was determined as a function of composition at 650°C. The electrical conductivity increased with increasing the mole fraction of sulfur. The temperature dependence of the electrical conductivity of Ni3S2 was measured between 50 and 750°C and found to exhibit metallic behavior.

Electronic Resource

1573-4889

diffusion ; surface potential ; work function

Springer Online Journal Archives 1860-2000

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Abstract The reequilibration kinetics (redox kinetics) of oxides are discussed in terms of the point defects in nonstoichiometric crystals, and the role of the surface and near surface layer are emphasized. Segregation of a component preferentially to the surface may occur. This very thin segregated layer may change the near surface composition, structure, and electrical properties with a consequent effect on the reequilibration kinetics.

Electronic Resource

1573-4889

work function ; contact potential

Springer Online Journal Archives 1860-2000

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Abstract The work function for PbS, PbSe, and PbTe has been measured in the temperature range of 100–350° C in oxygen and in vacuo. It was established that oxygen increases the work function for both PbS and PbSe and decreases the work function for PbTe. It was concluded that the process of oxygen chemisorption is followed by the formation of a heterophase layer. According to thermodynamic considerations, PbSO4, PbSeO4, and PbTeO4 are the most stable compounds as a reaction product of the oxidation of lead salts under the conditions studied.

Electronic Resource