G. Simkovich

Electrical conductivity and seebeck voltage of Fe2O3, pure and doped, as a function of temperature and oxygen pressure

Abstract The electrical conductivity and Seebeck voltage were measured on 99.9997% Fe 2 O 3 as a function of temperature and oxygen pressure, and on donor- and acceptor-doped samples as a function of temperature. The results indicate Fe 2 O 3 is an intrinsic semiconductor above 650°C, where the conductivity is described by σ=9191.2 exp (−1.11 eV /kT)(1/Ω cm ) and it is oxygen-pressure independent. The Seebeck voltage shows Fe 2 O 3 is n-type below 800°C and p-type above this temperature. Conductivity measurements on doped samples were used to calculate the carrier mobilities and these are given by the expressions: μ ( electrons )=( 1998 T ) exp (−0.17 eV /kT)( cm 2 Vs ) , μ ( holes )= ( 9298 T ) exp (−0.29 eV /kT)( cm 2 / Vs ) . The electrons are the most mobile carrier below ≈800°C but the hole is more mobile above 800°C, and this probably explains the conversion from n- to p-type behavior. The concentration of electrons is greater than that of the hole above 650°C, and the carrier concentration product is given by ( np )= 1.34×10 42 exp (−0.78 eV/ kT ). The appropriate defect equation for intrinsic Fe 2 O 3 is 0= n + p and the formation expressions for the minor atomic defects are Fe 2 O 3 →2( Fe int N+ )+2N n + 3 2 O 2 , where N =2 or 3 and Fe 2 O 3 →2( Fe x Fe +3 V O ⋯ +6 n + 3 2 O 2 .

Production of pyrrhotites by pyrite reduction

Abstract Research in coal conversion, aimed at gaining an understanding of the catalytic and magnetic properties of iron sulphides found in coal, could benefit from studying well-defined materials. A method is given for the production of pyrrhotites of known stoichiometry by the reduction of pyrite. Theoretical calculations are provided for this reduction by H 2 and CO. Experimental results, confirmed by X-ray analysis, are given for the H 2 reduction of pyrite to form three different pyrrhotites in the temperature range 400–500 °C.

Electrical conductivity and point defect behavior in ceria-stabilized zirconia

Abstract Ceria-stabilized zirconia has tremendous potential as a thermal barrier coating in gas turbines and as a fuel cell material. Unlike most stabilized-zirconia systems which are ionic conductors, ceria-doped zirconia has exhibited large electronic contributions at moderate temperatures and oxygen activities. The focus of this research has been to characterize this mixed conduction behavior. Electrical conductivity measurements have been performed as a function of composition, temperature and oxygen partial pressures. At high oxygen activities, oxygen vacancies are the dominant charge carriers while at lower oxygen partial pressures an electron hopping mechanism dominates. Reduced cerium ions and Ce 3+ -oxygen vacancy associates generate these conducting electrons. Kroger-Vink diagrams have been constructed which detail these mechanisms and describe the point defect behavior for ceria-stabilized zirconia.

Rapid oxidation of liquid tin and its alloys at 600 to 800°C

The behavior of liquid tin and its alloys in oxygen at temperature range 600 to 800°C were investigated. Rapid and nearly linear reaction kinetics were observed for pure tin at temperature higher than 700°C. Marker experiments, which determine the mode of mass transport through the scale, and wetting phenomena between the oxide and melts were studied to delineate the reaction mechanism of oxide growth. Moreover, the rates of oxidation of tin were markedly changed by alloying it with small amount of foreign elements. Significantly increased oxidation rates for binary tin alloys containing Mg, Ba, La or Ca were observed. TEM studies indicated that additional growth stresses were introduced into the SnO2 scales by these additions.

The diffusion coefficient of carbon in cementite, Fe3C, at 450°C

Abstract The rate of formation of Fe3C on fine iron powders at various carbon activities set by flowing CH4/H2 gas mixtures was determined at 723 K utilizing an automatic recording semimicro balance. The rate of growth of the Fe3C was primarily parabolic. From the kinetics of formation and taking some freedoms with the geometries involved estimates of the diffusion coefficient of carbon in Fe3C were made and were found to be in the range of 10−16 to 10−15 cm2s−1 Because the diffusion coefficient of carbon increased somewhat as the carbon activity increased it was tentatively assumed that the carbon moves via an interstitial or interstitialcy mechanism.

High-temperature sulfidation behavior of iron-based alloys in hydrogen sulfide-hydrogen gas mixtures

Sulfidation of Fe-Cr binary alloys, Fe-Cr-Al ternary alloys, and commercial stainless steels has been carried out at 540°C in a H2S-H2 gas mixture of 0.7 vol.% H2S and a total pressure of 1 atm. Sample exposure time was from 3 to 5 days. Gravimetric, metallographic, electron microprobe, and x-ray studies were made on a few selected alloys. Sulfidation rates of Fe-Cr-Al alloys were generally slower than corresponding Fe-Cr binary alloys and in many cases were orders of magnitude slower. Scales were extensively dual-layered and showed Al and Cr enrichment at the alloy-scale interface. Tests on the commercial stainless steels confirmed the results obtained with the laboratoryprepared Fe-Cr-Al alloys.

Electrochemical studies on lead iodide

Abstract The total d.c. electrical conductivity of undoped PbI2 was measured as a function of iodine potential in the region 150–300°C to determine the nature of charge carriers in PbI2. Results indicate that PbI2 is an ionic conductor with electron holes as the minority carriers in this temperature range. Electrical polarization experiments were also performed to determine the electron hole conductivity and concentrations, mobilities and chemical diffusion coefficients of electron holes in undoped PbI2 between 150 and 300°C.

The change in growth mechanism of scales due to reactive elements

An analysis of the effect of reactive elements, or their oxides, upon high temperature-oxidation behavior of chromia-forming alloys is presented in this note. The change in mechanism for the growth of “pure” chromia, cation motion in the scale, to that formed on alloys containing the reactive elements, anion motion in the scale, is attributed to the fact that almost all reactiveelement oxides are, on the basis of ion motion, anionic conductors. Thus, the reactive-element oxide present as an adsorbed layer at the chromia grain interfaces as well as fine particles residing at these interfaces restricts cation motion and imposes anion diffusion along these primary paths of scale growth. The effect of scale adherence is not addressed.

Effect of solid state impurities on the dissolution of Nickel oxide

The influence of two dopants: Li2O and Cr2O3 on the dissolution of NiO has been investigated using dilute sulfuric acid solutions at 60 °C. The dissolution rate is decreased by chromium additions of up to 1.0 mol pct Cr2O3, but the addition of 1 mol pct Li2O dramatically increases the dissolution rate. The results are interpreted in terms of the modification of the defect structure of NiO (p-type semiconductor with metal vacancies) by the dopants. It is proposed that Li+ doping increases the surface concentration of Ni3+ and that subsequent strong Ni3+-SO42− interaction enhances NiO dissolution. The NiO-SO42− surface interaction is confirmed with electrophoretic mobility measurements. A similar, though less pronounced, trend is observed when perchloric acid is used instead of sulfuric acid as the dissolution reagent. The difference in the behavior of the acids is attributed to differences in their affinities for the oxide surface.

Formation of ZnO/metal composites by rapid oxidation of Zn melts

A novel ZnO-based composite material is fabricated by oxidation of Zn alloy melts, in which outward growth of the reaction products and rapid reaction kinetics are observed. Additions of Na and Bi to the Zn melts are found to be essential to increase the matrix growth rate to a practical level. Optimum reaction rates are observed for Zn-3Na-5Bi alloys oxidized at temperatures between 450 and 550°C in pure oxygen. Processing parameters, such as alloy composition and oxygen activity, also have a profound impact on the resulting microstructure and reaction kinetics. The evidences supported that the continuous outward growth of the matrix is achieved via modification of the ZnO defect structure and partial wetting of the growing oxide by the melts.