Some electrical transport studies on compounds of the Tl-Te system

Galvanomagnetic, thermoelectric and electrical conductivity measurements were made on single crystal specimens of Tl₂Te₃, TlTe and -phase in the temperature range from 77° K up to 500 to 700° K. All crystals were found to be of p-type conductivity. Measurements of the electrical conductivity versus composition have revealed that -phase represents solid solutions of Tl and Te in the compound with formula Tl₅Te₃. Also, it has been found that both TlTe and -phase samples exhibit a metallic behaviour in contrast to Tl₂Te₃, which has semiconducting properties. In the intrinsic conduction region a thermal energy gap of a 0.68 ± 0.03 eV was found, which was compared with that obtained from optical transmission measurements. The electrical properties of Tl-Te compounds are discussed on the basis of the general valence rule.     

Microstructural characterization of as-cast high-nitrogen Fe-15Cr-15Ni alloys

A base composition of Fe-15Cr-15Ni was melted in a hot-isostatic-pressure furnace under nitrogen pressures from 0.1–150 MPa (1–1500 atm) to produce materials with nitrogen concentrations from 0.04–3.6 wt%. Microstructural characterization of the as-cast materials was completed using optical microscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy techniques. The phases present in the materials were austenite, nitrides (Cr₂N and CrN), and martensite. All of the materials solidified with primary austenite dendrites. Dendritic solidification of CrN occurred in interdendritic regions of the austenite in alloys containing between 0.6 and 1.9 wt % N. Cellular precipitation of Cr₂N occurred in alloys with intermediate nitrogen concentrations (0.6 and 1.0 wt %). The orientation relationship between the Cr₂N and austenite, expressed as {1 1 1}y{0 0 0 1}Cr₂N and 1 1 0 –1 1 0 0Cr₂N, was confirmed. Precipitates of CrN with both lamellar and disc morphologies formed in the austenite. Both had a cube-cube orientation relationship with the austenite. Only about 10% of the 3.6 wt% N material solidified as primary austenite. The remainder of the material solidified as a eutectic according to the reaction L +CrN. All of the austenite in the eutectic region subsequently transformed to martensite. Increasing volume fractions of martensite formed with increasing bulk nitrogen concentrations in the materials due to alloy depletion of austenite caused by nitride formation.     

Hydrazine method of synthesis of γ-Fe2O3 useful in ferrites preparation. Part III – study of hydrogen iron oxide phase in γ-Fe2O3

Direct current electrical conductivity () measurements as a function of temperature have been carried out on -Fe₂O₃ prepared from precursors, iron (II) carboxylatohydrazinates, -FeOOH and hydrazinated -FeOOH. The conductivity variation obeys an Arrhenius equation, _I = \_oe^- E / kT and the plots of log versus 1/T of the as prepared -Fe₂O₃, which are in general linear, during the very first heating up to 350°C and cooling to room temperature (RT) do not overlap. This indicates a hysteresis behavior of conductivity, thereby suggesting involvement of two different conductivity mechanisms. When the heat treated sample was equilibrated in a known partial pressure of moisture at 200°C and then conductivity measured from RT, the log plots during heating and cooling did not overlap and a hysteresis behavior similar to the as prepared -Fe₂O₃ is observed again in the conductivity. Water is considered to be crucial during the synthesis of -Fe₂O₃ through magnetite, Fe₃O₄. Protons, H⁺, are thought to be introduced in the spinel Fe₃O₄ making it defective and the oxidation product of this is -Fe₂O₃ which retains few protons in its spinel structure. From the structural similarity of such proton incorporated -Fe₂O₃ and lithium ferrite, LiFe₅O₈, (Fe³⁺)₈ [Fe³⁺ ₁₂ Li¹⁺ ₄]O₃₂, a formula HFe₅O₈, (Fe³⁺)₈ [Fe³+₁₂H¹+₄]O₃₂ is suggested. A hydrogen iron oxide of formula H_1-xFe_5+x3O₈, where x 0.1 is probably formed as a maximum limit. Protons are removed during the very first heating of the as prepared sample in the present studies and hence the conductivity of proton free -Fe₂O₃ is different and therefore a hysteresis behavior is observed. Moisture equilibration reintroduces the protons. The lithiated samples in the present studies were found to substitute for protons in -Fe₂O₃ and no hysteresis behavior is observed in such samples even after moisture equilibration.     

Spatial structure of the superconducting phase in heavy-fermion systems and anomalies in the excitation spectrum

A degenerate superconducting phase in a metal with strong spin-orbital coupling may have a domain structure at low temperatures. The domain walls (of width =v_F/₀) carry a finite density of states N(0)mk _f/² per unit volume, which in turn results in the presence of a linear term T in the electronic specific heatC _e (T). With the help of the Atiyah-Singer theorem, this can be proved for a wide class of domain structures.     

Effect of gamma irradiation and silver doping on YBCO superconductor

The effect of -irradiation and silver doping on the properties of YBCO superconductors has been studied by electrical resistance measurements and x-ray diffraction techniques. The zero-resistance superconducting transition temperature of (YBCO)_1–xAg _x systematically increase by about 2 K with an increase of Ag content up tox=0.04. Further increase in Ag content decreases the transition temperature. The normal-state resistance decreases up to a factor of 3.4 with an increase of Ag content. No significant change in the transition width has been observed due to silver addition up tox=0.30. Undoped YBCO and the Ag-doped specimens of (YBCO)_1.96Ag_0.04 were irradiated with-rays up to an integrated-dose of 77 MR. The normal-state resistance of the specimens gradually increases and the transition temperature slightly decreases with increase of-dose. The intensities of the x-ray diffraction peaks of the monolayers of the sample powders decrease with -irradiation without noticeable change of diffraction angles or peak widths. These observations indicate that the high-T _c supconductor YBCO is quite sensitive to-irradiation but drastic lattice expansion does not occur up to a-dose of 77 MR.     

Electrical conductivity and optical properties of gamma-irradiated niobium phosphate glasses

A detailed study of the d. c. electrical conductivity and the optical absorption spectra as a function of gamma-irradiation doses for prepared (20 mole% Nb₂O₅ –80 mole% P₂O₅) glass system is presented. The temperature dependence on the d. c. electrical conductivity measurements is studied over the temperature range from room temperature to 573 K. The induced changes in the d. c. electrical conductivity caused by different doses of gamma irradiation (0–16 Mrad) were studied. The electrical conductivity, , and the activation energy, E, values were found to be sensitive to the gamma-ray doses. The optical absorption spectra were measured in the wavelength range from 200 to 1100 nm at different -doses, in the range from 0 to 28 Mrad. The obtained results are analysed assuming optical absorption by indirect transition. Values of the absorption coefficient, , the optical energy gap, E _opt, and the width of the band tail, E, are found to be -irradiation dose dependent.     

Electrical conduction in γ-irradiated and unirradiated zinc-iron ferrites

The electrical conductivity of -irradiated and unirradiated finely-divided spinels of composition (Zn _x ²⁺ Fe _1–x ²⁺ Fe ₂ ³⁺ )O ₄ ^2– was studied in a nitrogen atmosphere as a function of temperature. The results of both -irradiated and unirradiated ferrites with compositions x 0.79 showed that the electrical conduction occurred by fast electron exchange amongst Fe²⁺ and Fe³⁺ ions situated on octahedral sites in the spinel lattice, by a hopping mechanism. For composition x > 0.79 it was found that the transition of the charge carriers through cation vacancies is the predominant mechanism. Some breaks in the conductivity-temperature curves were found near the Curie points. Seebeck voltage measurements showed that -irradiated and unirradiated spinels behave as n-type semiconductors. The effect of -irradiation on the conductivity, activation energy, charge carriers and the conduction mechanism is discussed.     

Electrical and optical studies on γ-irradiated pure and chromium-chloride-doped polyvinyl alcohol

Measurements of optical absorption, electrical conductivity and of the dielectric constant were used to study the changes that occur due to -radiolysis of pure and CrCl₃-doped polyvinyl alcohol (PVA) thin films. Identification of the structure and assignments of energy bands were derived in terms of ligand field theory. Induced changes in the absorbance at 282 and 222 nm bands in pure and doped PVA samples, respectively, appeared to be dose dependent. The direct-current (d.c.) electrical conductivity of the samples was measured in the temperature range 296–433 K, and the results obtained revealed that -irradiation enhances the conductivity. The calculated activation energy in the lower temperature region also proved to be dose dependent. Plots of the dielectric constant, , against temperature show an interesting behaviour which depends on the dopant concentrations and exposure to -irradiation. The results obtained suggest that these materials may have an application in dosimetry.     

Morphological and crystallographic features of γ-γ′ Ni-Al and Ni-Al-Ti two-phase bicrystals

Several- Ni-Al and Ni-Al-Ti two-phase bicrystals were made by the solid-state diffusion couple method. Each couple consisted of a-phase single crystal and a pure-Ni polycrystal, and was annealed at 1473 K in an Ar gas atmosphere. Single crystal layers of-phase with uniform thickness always grow into the parent-phase single crystals. The resultant/ interface has no voids or facets regardless of the orientation of interface and the chemical composition of the-phase. Porosity formation due to the Kirkendall effect is observed in the diffused region. Concentration profiles exhibit nearly constant gradients in-phase. The orientation relationship between both phases is found to be 001//001, that is, the-phase grows epitaxially along the crystal orientation of-phase.     

A high-resolution-electron-microscopy study of a γ/γ′-α directionally solidified eutectic alloy

The microstructure of a /- directionally solidified (DS) eutectic alloy with a nominal composition of Ni-30.26Mo-6.08Al-1.43V (wt%) was investigated by means of high-resolution electron microscopy (HREM) and analytical electron microscopy. The -fibres exhibited a typical morphology with a rectangular cross-section and they displayed the Bain orientation relationship (OR) with the / matrix; that is, [001][001] and (110)(010). Misfit dislocations and lattice strain fields existed at the / interface for different habit planes; that is, (110)(010) and (100)(110) were analysed. EDAX (Energy dispersive X-ray) analysis showed that the composition of the -phase was approximately Ni₄(Mo, Al, V); it contained 90° rotational domains of Ni₃(Mo, Al, V) with a DO₂₂ structure and Ni₂(Mo, Al, V) with a Pt₂Mo structure.     

Electrical conduction in γ-irradiated and unirradiated Fe3O4, CdFe2O4 and Co x ZN1−x Fe2O4 (0 ⩽x ⩽ 1) ferrites

The electrical conductivity of -irradiated and unirradiated finely divided ferrites of composition Fe₃O₄, CdFe₂O₄ and Co _x Fn_1–x Fe₂O₄(0 x 1) was studied in a nitrogen atmosphere as a function of temperature. Fe₃O₄, ZnFe₂O₄ and CdFe₂O₄ showed n-type conduction, whereas CoFe₂O₄ showed p-type conduction. For Co _x Zn_1-x Fe₂O₄ it was found that the type of conduction varies with the composition of ferrites. The electrical conduction in Fe₃O₄, and Co _x Zn_1–x Fe₂O₄(0 x 1) was explained by a hopping mechanism, whereas the conduction in ZnFe₂O₄ and in CdFe₂O₄ is interpreted on the basis of the transfer of charge carriers through cation vacancies present on octahedral sites. The effect of -irradiation on the conductivity, activation energy, charge carriers and the conduction mechanism is discussed.     

The influence of ageing on the stability of β-γ/γ′ derived microstructures in Ni-Al-Cr-(Co) alloys

Multiphase Ni-Al-(Fe)-(Cr)-(Co)-based intermetallics with (B2)- (A1)/(L1₂), - or - microstructures can exhibit significant room-temperature tensile ductility. In the case of Ni-Al-Cr-based alloys, microstructural development is complicated by the precipitation of -Cr, which can supplant the -phase during ageing of three-phase -/ microstructures. An investigation of the stability, during ageing, of cast Ni-Al-Cr-(Co) alloys with microstructures derived from -/ is reported. In the as-cast condition, the materials investigated consisted of a dendritic matrix containing L1₀ type martensite and interdendritic /. Extensive intra- and interdendritic -Cr precipitation was also observed. The stability during ageing of the interdendritic / microstructure is also considered and transformation of the L1₀ martensite is examined.     

The constitution of the Ni-Al-Ru system

The constitution of the Ni-Al-Ru system has been investigated in the range 0 to 50 at% Al. Isothermal sections at 1523 and 1273 K have been determined using microstructural observations, electron probe microanalysis and X-ray diffraction. The phases present were: nickel-based solid solution (); (based on Ni₃Al); solid solutions based on NiAl and RuAl, respectively (designated ₁ and ₂), and ruthenium-based solid solution (Ru). The maximum solubility of Ru in was 5 at%. ₁, and ₂ show extensive range of solubilities, namely up to 20at% Ru in ₁ and up to 25 to 35 at% Ni in ₂. Three-phase equilibrium between , ₂ and (Ru) existed at 1523 and 1273 K. Also at 1523 K, three-phase equilibria existed between , and ₁ and ,₁ and ₂, while at 1273 K, the equilibria were between , ₁,₂ and , , ₂ indicating the occurrence of a reaction +₁, +₂ at a temperature between 1523 and 1273 K. Liquidus features have been deduced from data on as-solidified structures. Lattice parameter data and hardnesses are also reported.     

Some magnetic properties of γ-Fe2O3 synthesized from ferrous fumarate half-hydrate

-Fe₂O₃ synthesized from ferrous fumarate half-hydrate was studied by measurements of D.c. electrical conductivity, Seebeck coefficient, initial magnetization and magnetic hysteresis, and by Mössbauer spectroscopy and scanning electron microscopy. The phase transformation observed by electrical conductivity measurements matched well with the phase transformation observed by the variation with temperature of initial magnetization measurements of -Fe₂O₃; this magnetic study also established the single-domain character of -Fe₂O₃. The magnetic hysteresis values of the -Fe₂O₃ synthesized indicated improved values over that of a -Fe₂O₃ sample synthesized by established procedures. The scanning electron micrographs showed that the -Fe₂O₃ particles were acicular in shape and the Mössbauer spectrum showed a well-resolved six-band spectrum. The presence of a hydrogen ferrite phase was also confirmed by the electrical and magnetic measurements.Deceased, 10 October, 1985.     

Li+ ion mobility in eucryptite phases

The two polymorphs of LiAlSiO₄, and eucryptite, were studied by X-ray and thermal methods. The-eucryptite originated from different precursors that obtained from LiA zeolite is presented. It is shown that the electric conductivity of-eucryptites increased as the unit cell parameter,c ₀, increased. These results correlate well with chemical compositions and thermal investigations. The conductivity in mixed systems depends on the ratio/ eucryptites which indicates a higher Li⁺ mobility in the-phase.     

Oxygen ion conduction in γ-Bi2O3 doped with Sb2O3

Electrical conduction in bcc-Bi₂O₃ doped with Sb₂O₃ was investigated by measuring electrical conductivity, as a function of temperature and oxygen partial pressure , and ionic transference number. The-Bi₂O₃ doped with 1 to 3 mol% Sb₂O₃ was stable up to 550° C and showed an oxygen ionic conduction in the region of 10⁵ to 10^–9 Pa. As the Sb₂O₃ content increased, ionic conductivity increased up to 2.5 mol % Sb₂O₃ (1.8×10^–3–1cm^–1 at 500° C) and then decreased. However, the activation energy for ionic conduction remained almost unchanged. It was proposed that the-Bi₂O₃ contains a lot of oxygen vacancies and incorporated Sb⁵⁺ ions at tetrahedral sites which affect the concentration of oxygen vacancy effective for conduction.     

Magnetic field dependence of the density of states of YBa2Cu3O6.95: Implications for the vortex structure

The total specific heat of YBa₂Cu₃O_6.95 single crystals includes contributions from phonons and spin-1/2 particles, as well as electronic contributions. The electronic specific heat is described by a quadratic term T² in zero field and a linear term [(0)+(H)]T which is increased when a magnetic field H is applied perpendicular to the CuO₂ planes. In agreement with d-wave superconductivity, we find that _n/T_c and (H)_n(H/H_c2)^1/2, where _n is the coefficient of the normal-state linear term. The H^1/2 dependence of the density of states at the Fermi level was predicted by G. Volovik for lines of nodes in the gap: the quasiparticles which contribute to this density of states are close to the nodes in momentum space and are located outside the vortex core.     

Chemical Transformations of γ-Al(OH)3during Closed-System Heat Treatment

The chemical transformations of -Al(OH)₃during closed-system heat treatment in a self-generated gaseous atmosphere (water vapor) were studied. At t 200°C, -Al(OH)₃was found to convert into -AlOOH, which, in turn, converts into the equilibrium phase -Al₂O₃at t 400°C. The processes underlying the effect of water vapor on the kinetics and mechanisms of the chemical transformations of -Al(OH)₃and -AlOOH in a closed system are discussed.     

Gamma-ray dosimetry using zinc phosphate glasses

A series of ZnO-P₂O₅ glass systems has been prepared by melting ZnO with anhydrous P₂O₅ in an open crucible. The temperature dependence on the d.c. electrical conductivity measurements for these glasses is studied over the temperature range from room temperature to 573 K. It was found that the d.c. conductivity decreases with increasing mol% ZnO. The induced changes in the d.c. electrical conductivity caused by different doses of gamma irradiations were also studied. Gamma doses up to 5.O Mrad were used, results were explained and discussed in terms of the interaction of gamma rays with the glass compositions. Results of the present work have shown that the effect of-ray on the d.c. conductivity of the glass system is composition dependent and the low ZnO concentration specimen is more sensitive to the-irradiations.     

Viscoelastic and rheological behavior of concentrated colloidal suspensions

Molecular approaches are discussed to the density (), viscoeleastic (), and rheological () behavior of the viscosity(,,) of concentrated colloidal suspensions with 0.3 < < 0.6, where, is the volume fraction, the applied frequency, and ; the shear rate. These theories are based on the calculation of the pair distribution functionP ₂(r,,), wherer is the relative position of a pair of colloidal particles. The linear viscoelastic behavior(,,=0) follows from an equation forP ₂(r,,) derived from the Smoluchowski equation for small, generalized to large by introducing the spatial ordering and (cage) diffusion typical for concentrated suspensions. The rheological behavior(,,=0) follows from an equation forP ₂(r,) of a dense hard-sphere fluid derived from the Liouville equation. This leads to a hard-sphere viscosity^hs(,) which yields the colloidal one(,) by the scaling relation(,) ₀=^hs(,) _B, where ₀ is the solvent viscosity. _B is the dilute hard-sphere (Boltzmann ) viscosity and the's are appropriately scaled,(,) and(,) agree well with experiment. A unified theore for(,,) is clearly needed and pursued.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994. Boulder, Colorado, U.S.A.