Electrical properties of growing alumina scales

Scales growing on platinum 22 wt.% aluminum specimens at 1100°C were investigated by the following electrical techniques: open-circuit potential and short-circuit current as a function of time and oxygen pressure; current-voltage relationship at several oxygen pressures; transients in the short-circuit current on rapidly changing the oxygen pressure; changes in the growth rate with applied potential. The growth characteristics of the scale were also investigated using kinetic and metallographic techniques. The apparent parabolic rate constant of oxidation decreases in the first hours of oxidation and stabilizes after 20–30 hr at 2 × 10^–13 g²·cm^–4·sec^–1. Transport in the scale is by lattice and grain-boundary diffusion and the changing kinetics are a consequence of changes in the oxide grain size. It was also determined that the electrical characteristics of the scale were affected by the hydrogen-water or carbon monoxide-carbon dioxide atmospheres used to obtain low oxygen pressures. Due to these complexities and to the difficulty in making a satisfactory oxide-gas electrode only qualitative observations could be made from the electrical measurements. These measurements are compatible with the scale having constant ionic conductivity in the pressure range 1 to 10^–15 aim and predominantly electronic conductivity at lower oxygen pressures. The rate of oxidation can be accelerated or retarded by passing a current through the scale.     

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.     

Dissolution kinetics of small amounts of oxygen in tantalum alloy T-111 and internal oxide displacement reactions during annealing

Oxygen was added to T-111 (Ta-8W-2Hf, wt.%) at 820 and 990°C at an oxygen pressure of about 3×10^–4 Torr (4×10^–2N/m²). The technique employed permitted predetermined and reproducible doping of T-111 up to 3.0 at% oxygen. Based on the temperature dependence of the doping reaction, it is concluded that the initial rates of oxygen pickup are probably controlled by solution of oxygen into the T-111 lattice. Although hafnium oxides are more stable than those of tantalum or tungsten, analyses of extracted residues indicate that the latter oxides predominate in the as-doped specimens, presumably because of the higher concentrations of tantalum and tungsten in the alloy. However, high-temperature annealing promotes gettering of dissolved oxygen and of other oxides to form hafnium oxides. Small amounts of tantalum and tungsten oxides were still present after high-temperature annealing. Tungsten oxide (WO₃) volatilizes slightly from the surface of T-111 at 990°C. The vaporization of WO₃ has no apparent affect on the doping reaction.     

Adsorption of oxygen on steel

A study of the adsorption of oxygen on steel at room temperature has been performed in an ultra-high-vacuum system with oxygen in the pressure range from 10^–9 to 10^–4 Torr. Ellipsometry, surface potential difference, and Auger spectroscopy were the surface techniques used. The surface of the steel was cleaned by Cs⁺ and Ar⁺ bombardment. The mechanism of the adsorption process is postulated to involve adsorption of molecular oxygen, followed by dissociation and chemisorption of atomic oxygen to form one monolayer. Further adsorption on top of this monolayer is associated with a place-exchange process to produce multilayers of oxide.This project was supported by the Research Department of the Rocketdyne Division of Rockwell International.     

Low-pressure,high-temperature siliconizing of tungsten-I

The study of surface siliconizing of tungsten ribbons by low pressure (10^–3 to 10^–6 Torr) and high temperature (800 to 1200°C) heterogeneous decomposition of SiH₄ shows a competition between silicon deposition on the surface and its diffusion in the metal Coatings of W₅Si₃ and WSi₂ are formed which grow parabolically or linearly depending on deposition and temperature conditions; their thickness has been determined without destroying the ribbon by measuring the variations of the electrical conductivity with the quantity of silicon introduced. The activation energy of the inter diffusion coefficients, the composition and resistivity of the phases, and the surface parameters corresponding to the surface evolution (monochromatic and total emissive powers) have been determined for the different compounds. The decomposition kinetics of SiH₄ were also determined.     

The influence of surface preparation on the structures of nickel oxide formed on the (100) face of nickel

A study has been made on the influence of surface preparation involving abrasion, electropolishing, and vacuum annealing on the physical nature of the (100) nickel crystal face and on the morphological development of the oxide film. The structure of the initial surface was dependent upon the method of surface preparation, but electropolished specimens subjected to conventional or high vacuum anneals at 10^–6 and 10^–10 Torr, respectively, and temperatures of 800 and 700° C were structurally identical. Thermal faceting of surfaces prepared to correspond to the (100) plane was negligible. Oxidation at 500° C and over the oxygen pressure range 10^–5–400 Torr led to formation of polycrystalline oxide crystallites randomly distributed over the surface during early stage exposures; the crystallites were only a few hundred Angstroms in size upon coverage of the surface by a nickel oxide film. These films were relatively uniform for thicknesses up to 2000 Å. Crystallite growth processes led to three major epitaxial relationships between oxide and metal: (100) _NiO , (111) _NiO , and (211) _NiO (100) _Ni .This work forms part of a research program sponsored by the National Research Council of Canada.     

Preparation of polyaniline–titanium dioxide hybrid materials in supercritical CO2

Polyaniline–titanium dioxide (PANI–TiO₂) hybrid materials were synthesized in supercritical CO₂ using two different methods. In the first method, separately synthesized TiO₂ particles were mixed with aniline to perform polymerization in supercritical CO₂. The second method included the preparation of aniline–TiO₂ hybrids through a sol–gel reaction of titanium isopropoxide in the presence of aniline. Further polymerization of aniline–TiO₂ hybrids in supercritical CO₂ produced PANI–TiO₂ hybrid particles. The final products showed the intrusion of PANI into the internal structure of TiO₂. The PANI–TiO₂ hybrid materials were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), thermogravimetric analysis (TGA), electrical conductivity (EC), Fourier-transform infrared (FT-IR) and X-ray diffraction (XRD) measurements. PANI–TiO₂ nano-composites synthesized with the first method showed a relatively low electrical conductivity of 3.78 × 10⁻² S/cm at 20 °C. TGA suggested that the particles contained 40.6% TiO₂ by mass and showed a strong interaction at the interface of TiO₂ and PANI. The electrical conductivity of the hybrid particles produced using the second method increased to 7.75 × 10⁻² S/cm and the TGA results showed 34.4% TiO₂ by mass. Through SEM and TEM analyses it was confirmed that the PANI has been interpenetrated into the three-dimensional network of the TiO₂ when the second method was used.     

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.     

Defect structure of NiO and rates and mechanisms of formation from atomic oxygen and nickel

The oxidation of nickel by atomic oxygen at pressures from 6×10^–3 to 0.33 Torr between 1050 and 1250 K has been investigated. In these ranges, the oxidation was found to follow the parabolic rate law, viz.k _p = 1.14×10^–5 exp(–13410/T)g² cm^–4sec^–1 for films of greater than 1 m thickness and was pressure-independent. The activation enthalpy for the oxidation reaction was 27±3 kcal mole^–1. Of a number of possible mechanisms and defect structures considered, it was shown that, based on reaction activation enthalpies, impurity effects, pressure independence, and magnitudes of the rates, the most likely was a saturated surface defect model for atomic oxidation. A possible model judged somewhat less likely was one having equilibrium concentrations of doubly ionized cationic defects rate-controlling in both atomic and molecular oxygen. From comparisons of the appropriate processes, the following enthalpy values were derived: H* (Ni diffusion in NiO) = 26.5 ± 8 kcal mole^–1 and H _f ⁰ (doubly ionized cation vacancies in NiO from atomic oxygen) = – 2.1 ± 6.0 kcal mole^–1. The recombination coefficient of atomic oxygen on oxidized nickel was determined to be 0.14 ± 0.06 in the temperature range 985 to 1100 K.     

AC electrical resistance measurements of PdHx samples versus composition x

Electrical and optical properties of SnO₂ film deposited on indium tin oxide substrate by dip coating have been investigated. The electrical conductivity of the SnO₂ film exhibits the extrinsic conductivity mechanism with two shallow and deep donor levels. The current voltage characteristics of SnO₂ film confirm the presence of space charge limited conduction. The density of states at the Fermi level N(E_F) for the SnO₂ film was determined using current–voltage characteristics and was found to be 1.63 × 10¹⁶ eV⁻¹ m⁻³. The Seebeck coefficient of the SnO₂ at room temperature indicates the n-type electrical conductivity. The Seebeck coefficient suggests that the electrical conductivity of the SnO₂ film varies from n-type conductivity to p-type conductivity with increasing temperature. The optical constants, such refractive index as dielectric constants, were determined from the reflectance, transmittance and absorption spectra. The refractive index dispersion behavior of the SnO₂ film obeys the single oscillator model. The fundamental absorption edge in SnO₂ film is formed by the direct allowed transitions with band gap of 3.40 eV.     

Electrical behaviour of hydridofluorides of potassium–calcium KCaH3−xFx with x = 1, 1.5, 2, 2.5

This paper reports the results of the electrical conductivity measurements for KCaH_3−xF_x series with (x = 1, 1.5, 2, 2.5) in the temperature range 298–503 K.The activation energy of the electrical conductivity for the studied compounds depends on hydrogen amount and Reau's criteria. Differential thermal analysis curves were measured in the same temperature range 298–503 K.Possible correspondence between preferential order given by X-ray diffraction, thermal behaviour and electrical properties are discussed.     

The diffusion of oxygen in Zirconia as a function of oxygen pressure

Diffusion of oxygen in monodinic zirconia was studied using the oxygen-18 gas-solid exchange technique. The zirconia was in the form of spheres prepared by dropping the powdered material through a high temperature plasma. Diffusion was measured as a function of equivalent oxygen pressure, using oxygen and CO-CO₂ mixtures, over the range 1 to 10^–21 atm. In pure oxygen at 700 Torr and between 600 and 1000°C, the diffusion is described by the equation,D =2.34×10^–2 exp — (45,300±1200)/RT. Experiments in CO-CO₂ mixtures at 850°C gave self-diffusion coefficients that showed a slight decrease with a decrease in D from 10^–6 atm, and finally a decrease inD from 10^–19 to 10^–21 atm. The behavior ofD with appears to be in agreement with a defect model involving anti-Frenkel imperfections, i.e., oxygen interstitials and anion vacancies, if it is assumed that the mobility for oxygen interstitials is greater than that for oxygen vacancies.     

A phenalenyl-based neutral stable π-conjugated polyradical

The first neutral π-conjugated polyradical based on phenalenyl with the spin unit inside the main chain was prepared. Cyclic voltammetry studies showed that this polyradical had excellent redox reversibility and enhanced π-delocalization between the neighboring spin units, in consistent with the UV–vis results. Initial magnetic studies showed that a strong antiferromagnetic interaction existed at solid state. The conductivity of the polyradical was measured with a value σ_RT ≈ 10⁻⁸ S/cm.     

Electrochemical performances of Ag–(Bi2O3)0.75(Y2O3)0.25 composite cathodes

The electrochemical performances of Ag–Y_0.25Bi_0.75O_1.5 (YSB) composite cathodes on Ce_0.8Sm_0.2O_1.9 (SDC) electrolytes have been investigated at intermediate temperature using AC impedance spectroscopy. The results indicated that the electrochemical performances of these composites are quite sensitive to the compositions and the microstructures of the cathode. The optimum YSB addition to Ag resulted in 10 times lower area specific resistance. The ASR of Ag-50 vol.% YSB was about 0.12 Ωcm² at 700 °C as compared to 3.9 Ωcm² for Ag cathodes. The observed high performance of Ag–YSB composite cathodes might be due to the high oxygen-ion conductivity of YSB and its high catalytic activity for oxygen reduction.     

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.     

Investigation of high-temperature oxidation of lanthanum hexaboride in oxygen and the effect of internal oxidation on the protective properties of the scale

The vacuum microbalance method, as well as petrographic and x-ray phase analysis were employed to investigate the oxidation of lanthanum hexaboride as a function of temperature (700–1200°C), oxygen pressure (0.1–740 Torr) and porosity of the sample. It has been established that LaB₆ is highly resistant to oxidation in oxygen up to a temperature of 1200°C. The protective properties of the scale are determined by the peculiarities of the process of internal oxidation, during which a lower lanthanum boride, LaB₄, was observed to be enveloped by the vitreous oxide phase, B₂O₃.     

Complex -phases in the Al–Pd-transition–metal systems: Towards a combination of an electrical conductor with a thermal insulator

-Phases in the Al–Pd–(Mn,Fe,Co,Rh,…) alloy systems form in wide compositional ranges and belong to the interesting class of complex intermetallics that are characterized by giant unit cells with quasicrystals-like cluster substructure. In order to see how the exceptional structural complexity and the coexistence of two competing physical length scales affect the physical properties of the material, we performed investigation of the magnetic, electrical, thermal transport and thermoelectric properties of the -phases in the Al–Pd–Fe, Al–Pd–Co and Al–Pd–Rh systems. Magnetic measurements reveal that the materials are diamagnetic, containing tiny fractions of magnetic transition–metal atoms (of the order 10–100 ppm). Electrical resistivity is moderate, of the order 10² μΩ cm, and shows weak temperature dependence (in most cases of a few %) in the investigated temperature range 4–300 K. An interesting feature of the -phases is their low thermal conductivity, which is at room temperature comparable to that of thermal insulators amorphous SiO₂ and Zr/YO₂ ceramics. While SiO₂ and Zr/YO₂ are also electrical insulators, -phases exhibit electrical conductivity typical of metallic alloys, so that they offer an interesting combination of an electrical conductor with a thermal insulator. The reason for the weak thermal conductivity of the -phases appears to be structural: large and heavy atomic clusters of icosahedral symmetry in the giant unit cell prevent the propagation of extended phonons, so that the lattice can no more efficiently participate in the heat transport. The thermoelectric power of the investigated -phase families is small, so that these materials do not appear promising candidates for the thermoelectric application.     

Thermophysical properties of lanthanum zirconate coating prepared by plasma spraying and the influence of post-annealing

Nano-scaled lanthanum zirconate powder prepared by co-precipitation–calcination method was plasma-sprayed into a thick coating on an alloy substrate. We investigated the thermophysical properties of the free-standing coating, including thermal conductivity and thermal expansion coefficients (TECs). Minimum value of the thermal conductivity (at 900 °C) of the coating was about 0.73 W m⁻¹ K⁻¹, and the average TEC in the measurement range was about 9.45 × 10⁻⁶ K⁻¹. Although the TEC value was similar to that of the bulk material, the change tendency versus temperature was different. After annealing at 1300 or 1400 °C for 50 h, we found that the heat insulation performance of the coating decreased with the heat treatment temperature, while the hardness and fracture toughness increased. A peak of sudden decrease in TEC value can be observed in the curve, and with increasing temperature, the peak shifted to high-temperature direction.     

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.     

Influence of the oxygen content and the preparation method on the power factor of PrBaCo2O5+δ samples (0.54 ≤ δ ≤ 0.84)

In this work, we focus on the influence of the oxygen content and the preparation method on the power factor of different PrBaCo₂O_5+δ samples (0.54 ≤ δ ≤ 0.84). The samples have been initially synthesized by the Pechini method. Their oxygen content has been subsequently modified by annealing under argon/oxygen flow or by electrochemical oxidation/reduction. The oxygen stoichiometry has a high impact on the electrical conductivity and Seebeck coefficient of the resulting materials. Moreover, by adequately reducing their oxygen content while increasing their intergrain conductivity (by increasing the particle size and degree of sintering) the power factor of these samples can be drastically improved. The best result is shown by the PrBaCo₂O_5.54 sample, that was annealed at high temperature under argon flow, whose power factor is as high as 6.5 μW/K² cm⁻¹ at ∼135 K, more than two orders of magnitude higher than that shown by the initial PrBaCo₂O_5.76 reference sample.