Experimental study of NiO electrical conductivity changes under low oxygen pressures

The electrical conductivity of NiO was measured at 740°C in an oxygen pressure range of 10^–2 –1.3 Torr. By means of continuous recording, longtime experiments were performed. The results show that for any admittance of oxygen, the electrical conductivity initially increased and then decreased to its initial value. For pressures higher than 0.1 Torr the decrease of the signal was reduced and the time required to attain the initial value sometimes reached several days. These results suggest that the electrical conductivity changes may be considered as a transitory phenomenon connected to attaining gassolid equilibrium.     

Electronic transport in thermally grown Cr2O3

Electrical conductivity of thermally grown Cr₂O₃ has been measured as a function of temperature and over a range of oxygen partial pressures from that of air to that of the Cr/Cr₂O₃ equilibrium. The conductivity showed p-type behavior over the range of the present investigation. At temperatures above 1000°C, the conductivity values were independent of oxygen partial pressure and indicated intrinsic semiconductor behavior. The mobility of holes, determined by measuring conductivity at fixed compositions (i.e., fixed in Cr_2-O₃), increased with temperature. This behavior can be attributed to hopping-type conduction. For 10^–5, the activation energy for hole hopping was 0.248 eV, and the calculated hole mobilities were 5.4x10^–2 and 2.4x10^–1 V/cm² · s at 500 and 1000°C, respectively. The oxidation kinetics of Cr were determined by measuring the electrical conductivity and electromotive force across the oxide layer at 875°C. The result agreed well with the oxidation data obtained in thermogravimetric tests.     

The oxidation behavior of a Zr-0.5Y alloy

A Zr-0.5 Y alloy was found to oxidize about 6 times faster than pure zirconium over the temperature range of 400 to 565°C. The activation energies were nearly identical (32 kcal/mole). The activation energies correspond to grain boundary diffusion of oxygen through the scale. The higher oxidation rate of the alloy was attributed to a higher anion vacancy concentration and the assumption that diffusion sites in the lattice and boundaries were in local equilibrium. Measurements on yttria-doped zirconia showed that ionic conductivity was increased markedly by yttrium and extended over a wide range of oxygen pressure. The defect structure of the doped oxide was changed to one of oxygen vacancies, even at the high end of the oxygen pressure range, 10^–8 to 0.2 atm, over which pure ziconia contains oxygen interstitials. The doped oxide was found to be extrinsic over the entire range of oxygen pressure and, although ionic conductivity predominated, electronic conductivity was still appreciable. The electronic conductivity, however, was still sufficiently high so that electron transport was not rate-controlling in the predominantly ionic-conducting scale.     

High-Temperature Oxidation of Ni–20 wt.% Cu from 700 to 1100°C

Ni–20 wt.% Cu was oxidized in different oxygen pressures from 1×10^–5 to 1 atm at 700–1100°C. The oxidation consisted of an initial transient period in which a composite scale of NiO and Cu oxides formed, and a subsequent quasi steady-state regime during which parabolic growth of NiO determined the overall oxidation rate. Based on the oxide composition and the oxygen- pressure dependence of the parabolic rate constant, it is concluded that outward transport of Ni via vacancies determines the growth rate of the oxide during the steady-state period, either in the grain boundaries or in the lattice. The influence of Cu dissolved in NiO on the oxidation rate and the oxygen-pressure dependence of the parabolic rate constants is discussed.     

Sulfur solubility in NiO and CoO at 1000°C

The solubility of sulfur in NiO and CoO at 1000°C has been investigated over a wide range of oxygen and sulfur partial pressures using CO, CO₂, and SO₂ as input gases. The concentration of dissolved sulfur increases regularly with sulfur partial pressure but appears to be insensitive to oxygen partial pressure. Dissolution of sulfur did not affect the electrical conductivity of NiO samples. It was concluded that sulfur probably dissolves, in the range 10^–2 –10^–3%, either by exchange with O^–2 ions occuping O^–2 sites as S^–2 ions, or as neutral sulfur on interstitial sites. The scatter in results prevented a more definite conclusion being drawn.     

A rapid method for determining parabolic rate constants of metal oxidation as a function of temperature and oxidant pressure

A new fast method is proposed for the determination of parabolic rate constants of metal oxidation as a function of pressure and temperature. The method consists of determining rate constants by oxidation of a single metal sample in a continuous manner, periodically changing the oxidant pressure or temperature. This method eliminates a number of errors inherent in the classical method which involves the use of a new metal specimen in each experiment and it further shortens the time of evaluating the functions k _p ^' = f (p, T). The method is particularly suitable for the determination of rate constants of slow processes. To verify the proposed method measurements of the kinetics of oxidation of cobalt and nickel at different oxygen pressures over the temperature range 1000–1300° C were carried out.     

High temperature oxidation of Nb-10 at. % W alloy

By means of the “interruption kinetic technique”, as applied to oxidation of tungsten under 0.048 bar O₂, the oxygen diffusion coefficient in growing WO_8−x has been determined for the temperature range of 568–908°C and may be expressed as: D = 6.83 × 10⁻² exp (−29,890/RT), with the activation energy given in cal/mole. Calculations are made to show the influence of temperature on the concentration of oxygen vacancies in WO_3−x, on their free energy of formation and on the ionic conductivity in WO_3−x. From the kinetic data of W oxidation at 800°C, prior to interruption-annealing, values of oxygen diffusion coefficients due to oxygen transport via lattice and short-circuit paths have been calculated as functions of time for growing WO_3−x. A simplified oxidation model is used for evaluation of the oxidation rate constant of W at 800°C.     

Oxidation of an Fe—19 wt. % Ni alloy in CO2 at 700–1000°C

The oxidation of an Fe—19.34 wt. % Ni alloy in dry CO₂ has been studied at 700—1000°C using thermogravimetry, metallography, and EPMA. Weight gains for oxygen consumption followed a linear-parabolic-linear sequence at all temperatures. During the initial linear stage the scale consisted mainly of magnetite and the activation energy of 133±25 kJ · mole^–1 is considered to be due to dissociation of CO₂ into CO and adsorbed oxygen on the outer magnetite surface. During the parabolic oxidation stage a continuous Ni-rich layer containing 70% Ni forms a barrier to the diffusion which has an activation energy of 192±79 kJ · mole^–1. The breakdown of the barrier layer causes a return to linear kinetics with an activation energy of 138±42 kJ · mole^–1 for dissociation of CO₂ on the outer surface. During the final linear stage there is pronounced general and intergranular subscale formation. Detailed information is presented of the Ni redistribution and concentrations during oxidation and its correlation with the kinetics and morphology.     

The electrical properties of niobium pentoxide scales during their formation on niobium

A study of the electrical properties of compact scales growing on niobium has revealed several new properties of the oxidation reaction. The growth of the oxide follows two parabolic rates, with the latter approximately four times the former. Both the rate constants and the time of transition between the rates are sensitive to oxygen pressure, but it is the latter property that largely explains the extreme sensitivity of the oxidation reaction to oxygen pressure reported previously. Concomitant with the change to the faster oxidation rate is the development of a voltage across the scale (metal negative). The voltage has a value of 0.3 V at 1 atm and decreases approximately linearly with the logarithm of pressure to 0 V at 10^–3 atm. The oxidation rate is essentially insensitive to induced changes in the voltage. These properties of the oxidation reaction cannot be explained by the usual formalization of point defect theory.This investigation was completed under the auspices of the National Research Council of Canada.     

Kinetics and morphological development of the oxide scale on iron at high temperatures in oxygen at low pressure

The oxidation properties of iron have been investigated at temperatures in the range 800–1000° C in oxygen over the pressure range 0.3–760 Torr. A duplex scale consisting of wustite and magnetite was formed during the earliest intervals examined. Hematite grew on the magnetite surface after an induction period which decreased with increasing oxygen pressure; this oxide developed as whiskers and platelets at temperatures less than 860° C and as small grains at higher temperatures. Iron transport occurs through the scale and involves short-circuit diffusion in the hematite layer. The oxidation kinetics obeyed a parabolic law independent of oxygen pressure since multilayer scale growth was directly dependent on the lattice diffusivity of iron and the iron gradient established in the wustite layer.This work was sponsored by the National Research Council of Canada.     

The oxidation of Ni-Nb alloys at low-oxygen pressures at 600–800°C

The oxidation of two Ni–Nb alloys containing 15 and 30 wt.% Nb has been studied at 600–800° C in H₂–CO₂ mixtures providing an oxygen pressure of 10^–24 atm at 600° C and 10^–20 atm O₂ at 700 and 800° C, these pressures being less than the dissociation pressure of nickel oxide. The scales formed on both alloys at 600 and 700° C show only a region of internal oxidation composed of a mixture of alpha nickel and niobium oxides (Nb₂O₅ or/and NbO₂), which formed from both the metal phases present, i.e., Ni₈Nb and Ni₃Nb. Only small, or even no, Nb depletion was observed in the alloys close to the interface with the zone of internal oxidation at these temperatures. On the contrary, samples of both alloys corroded at 800° C produced a continuous external scale of niobium oxides without internal oxidation. The corrosion mechanism of these alloys is examined with special reference to the effect of the low solubility of niobium in nickel.     

Effect of creep on the oxidation characteristics of Fe-Si alloys at 973–1073 K

Studies of the simultaneous creep and oxidation of Fe-1Si and Fe-4Si alloys at a constant tensile stress of 16 N· mm^–2 at 973–1073 K have shown that scales formed at oxygen partial pressures of 20–1013 mbar were thicker by a factor of 2 than those formed on uncrept specimens. Scales on uncrept alloys comprised alternate layers of wustite and fayalite, whereas scales on crept alloys exhibited an additional external layer of magnetite. Only intergranular oxidation (fayalite) was observed in uncrept alloys, but crept alloys showed both intra- and intergranular oxidation (silica). Uniquely nodular scales were formed only on the Fe-4Si alloy on crept and uncrept specimens. Oxidized, uncrept Fe-1Si showed a fine-grained ferrite substrate which was absent in the crept alloy. It is believed that oxide growth stresses stimulated a recrystallization process.     

Oxidation mechanism of titanium nitride in oxygen

Despite the apparent complexity of the linear oxidation of titanium or titanium nitride resulting from the formation of a layered structure of the rutile scale formed, the uniqueness of the mechanism is proven. The limiting step appeared to be the predominant short-circuit diffusion of oxygen (E 44 kcal · mole^–1) through a rutile lamella of variable thickness growing at the nitride-oxide boundary and fracturing periodically to form a detached porous layer through which molecular oxygen can penetrate. The pressure dependence of the diffusion process in the case of the nitride was associated with the outward migration of nitrogen, while the undulations of the kinetics under certain conditions were caused by the growth of a sintered, recrystallized outer zone of oxide. The periodic exfoliation of the oxide was related to its poor adherence to the substrate, certainly due to the presence at the nitride-oxide interface of a thin gray film probably composed of intermediate phases between TiN_x (or Ti) and TiO₂.     

The Role of Oxygen Uptake and Scale Formation on the Embrittlement of Vanadium Alloys

Vanadium alloys are of interest in fusion energy systems, however, their environmental durability is a major concern. Specimens of V–4Cr–4Ti were exposed to air and oxygen (10⁵,Pa), low pressure (10^–3–10^–6 Pa) oxygen and high purity He environments (10⁵–10¹ Pa) it at 500–700°C in order to characterize the surface oxide, determine oxidation kinetics and quantify effects on mechanical properties at 25 and 600°C. At low oxygen pressures (P_{O_2}.10^–5 Pa), linear reaction kinetics were measured for exposures up to 2000 hr and the data were used to develop a mathematical expression for the oxidation rate as a function of temperature and oxygen pressure. At higher pressures, linear–parabolic reaction kinetics were associated with high oxygen uptake and the formation of an external oxide layer. Room-temperature and 600°it C tensile ductility was reduced by these exposures, but specimens which formed an external oxide were found to retain some tensile ductility after exposure. However, similar specimens with an external oxide that were subsequently annealed for 2000 hr at 700°C became severely embrittled demonstrating that a surface oxide will not prevent degradation of this refractory alloy. Exposures in He were performed to determine the effect of total gas pressure on oxygen uptake.Dedicated to the Memory of Jackson H. DeVan.     

The ratios of layer thicknesses in a multiphase oxide scale

The diffusion controlled oxidation of a metal to produce two-layer oxides and one type of three-layer oxide is analyzed. It is proved that the relative thicknesses of the oxide layers must tend to a limiting constant value for all oxidizing conditions. The analysis for the three-layer oxide is applied to the oxidation of 9Cr steel in high pressure steam at 500°C. The proportion of hematite expected within the outer oxide layers produced in steam with oxygen partial pressures 10^–20 bar is calculated to be very dependent on the metal oxidation rate and not on the oxygen partial pressure.     

The direct observation and investigation of the oxidation of aluminum in the transmission electron microscope

The authors studied the oxidation of thin aluminum films free of oxide layers in situ prepared by evaporation directly in the electron microscope under ultra-high-vacuum conditions. The oxidation was realized at various temperatures (350–500°C) and at various oxygen pressures (1–10^–3 Pa). The formation and growth of the amorphous and crystalline products have been studied.     

Oxidation of γ-Ni−Al−Si alloys at 1073 K

The formation and development of oxides in Ni–4Al and Ni–4Al–xSi (at.%, x=1, 3, 5) alloys at 5–9×10^–6 and 1 atm oxygen pressure at 1073 K have been studied. The oxidation rate increased with an increase of silicon content in the alloy at the early stage of oxidation, but decreased after longer time exposure due to formation of an intermediate layer composed of NiO and spinel (NiAl₂O₄ and Ni₂SiO₄) between the top NiO layer and the internal-oxidation zone. This intermediate layer became a barrier for releasing stress, generated by the volume expansion associated with oxidation of solute atoms, resulting in high dislocation density and severe distortion in the internal-oxidation zone for the Ni–Al–Si alloys. In Ni–4Al alloy where no complete intermediate-layer formation occurred, stress was easily released by an enhanced vacancy gradient, and therefore an enhanced vacancy-injection rate into the alloy, resulting in a higher oxidation rate than the situation where a sample was oxidized at an oxygen pressure associated with the dissociation of NiO.     

Oxidation rates of niobium and tantalum alloys at low pressures

Niobium and tantalum alloys have excellent properties for use in high-temperature, space-power applications, but must be protected from oxidation that would result from exposure to air in ground-evaluation tests. The oxygenuptake/oxidation rates of three alloys, Nb-1Zr, PWC-11, and ASTAR-811C were measured at oxygen partial pressure of 10^–6 and 10^–7 torr at temperatures up to 1350 K. No visible oxide film was observed, and the oxidation rate was found to be linearly proportional to pressure and exponentially proportional temperature. A thin molybdenum coating on Nb–1Zr was a barrier to lowpressure oxidation at 773 K.     

Pilot plant scale preparation of dodecylbenzene sulfonic acid doped polyaniline in ethanol/water solution: Control of doping, reduction of purification time and of residues

Aiming to produce polyaniline doped with dodecylbenzene sulfonic acid on a pilot plant scale using a 10 L reactor, the synthesis was optimized on a bench scale using a 2³ factorial design, involving the variables: K parameter, DBSA/aniline molar ratio (Q) and final aniline concentration (C). The nominal yield and electrical conductivity were used as control parameters. An ethanol/water (2:5, v/v) solution replaced water as solvent. The samples were characterized by electrical conductivity measurements, infrared spectroscopy, X-ray diffraction, elemental analysis, thermogravimetry and density. The influence of the washing solvent volumes and storage time of oxidizer on the nominal yield and electrical conductivity of PAni/DBSA were also investigated. By using ethanol/water as solvent it was possible to reduce the filtration time, to eliminate the purification step and to control the content of DBSA in the polyaniline bulk, maintaining the principal characteristics, like electrical conductivity and density. The doped polymer was obtained with electrical conductivity in the range of 10⁻¹ to 10⁻³ S cm⁻¹, depending on the dopant concentration. Freshly purchased ammonium peroxydisulfate must be used for higher yields and better reproducibility.     

Kinetics and mechanism of iodide-film growth on lead-effect of short-circuiting and higher-valent dopant

The effect of a higher-valent dopant like Sb on the iodination rate of lead under normal and short-circuit conditions in iodine pressure of 0.615–6.578 kPa and in the temperature range of 423–523 K has been investigated. Like pure Pb, Sb-doped Pb also follows the parabolic law of film growth. The isothermal parabolic rate constants are found to be enhanced due to the presence of Sb. The iodine-vapor-pressure dependence of the isothermal parabolic rate constant has been observed to be k_ppI ₂ ^1/2 . This has been explained on the consideration of electron-hole migration across the film as the rate-limiting step. The activation energy for iodination of Sb-doped Pb under normal condition is estimated to be 64 kJ · mol^–1 in an iodine pressure of 0.615 kPa. The rate of iodide-film growth has been found to increase considerably under a short-circuit mode of experiments. Such observations have been explained with the concept of ion migration as the rate-limiting step for the film-growth process. The iodine pressure dependence of rate constants under short-circuit conditions is observed to be k_pI ₂ ^1/3 , associated with an activation energy value of 51 kJ mol^–1. The effect of putting additional resistances in series to the short-circuit Pt path during iodination of Sb-doped Pb is found to be similar to that observed for pure Pb. Results of the present study have been explained considering the prevalence of Schottky-Wagner type of point defects in the lead-iodide film. Wagner's electrochemical potential gradient has been confirmed to be the main driving force for the film-growth process. Iodide films have been characterized by SEM, EDS, EPMA, XRD, and AES analyses to substantiate the kinetics results.