Optical properties of coloured transparent alumina gel monolith glasses

Optical properties of transparent alumina prepared through gel process at room temperature and coloured with dopants have been studied. The absorption bands, molar absorptivity, effect of concentration and the dependence of oxidation state of copper on the copper content and water content are similar to the random network glasses. The ultraviolet absorption edge at lower energies confirm the disordered nature of the transition phase aluminasγ andδ. The absorption bands in the visible suggest the octahedral symmetry of oxygen around aluminum.     

Electrical conduction in the system Sr0·90La0·10Ti0·90M′0·10O3 (M′ = Co,Ni or Cr)

We have successfully synthesized the system Sr_{1 −x} La _x Ti_{1 −x} M′ _x O₃ where M′ = Cr, Ni and Co by using conventional solid state ceramic method. Powder X-ray diffraction patterns of the different compositions show the formation of single phase materials. Measurement of AC conductivity as a function of frequency at different temperatures in the range 300–550 K show that conduction in these compositions occurs due to hopping of charge carriers between localized transition metal ion sites.     

IR Radiative Properties of Solid and Liquid Alumina: Effects of Temperature and Gaseous Environment

An infrared spectrometer (spectral range 2–6 μm), coupled with a 9 μm pyrometer (Christiansen’s wavelength), has been developed to collect time-resolved measurements (32 spectra/s) of the spectral emissivity of alumina droplets (d≈ 3 mm) freely cooling in an aerodynamic levitation system from the liquid-to-solid phases [2800 → 1500 K]. The temperature and nature of the gaseous atmosphere surrounding the droplet (oxidizing/neutral/reducing) are two important parameters affecting the spectral emissivity in the semi-transparent range. Observations are discussed in the framework of the thermal activation and of the chemical interactions of alumina with the environment. Paper presented at the Seventh International Workshop on Subsecond Thermophysics, October 6–8, 2004, Orléans, France.     

Application of a geometric model to the hydrides of FeTi

Criteria of hole size and hydrogen-hydrogen distance have been used to develop rationales for the observed stoichiometries and preferred hydrogen sites in theα-,β ₁-,β ₂- andγ-phases of the FeTi-H system. For these four phases of FeTiH _n , the respective values ofn approximate 0, 1, 1.4 and 2. Structures and position vectors for the metal atoms were obtained from the literature for use in calculating the radii of the various interstices and the intersite distances in each phase. The model does not allow one to predict the nature of the structural transformations that occur in this system with increasing hydrogen concentrations, but knowing the metal atom positions in each phase does allow one to predict the preferred sites for hydrogen atoms. Most of the occupied interstices are octahedral sites coordinated by four titantium atoms and two iron atoms, but the reported occupation of a pseudo-octahedral site in FeTiD is explained within the framework of the model by allowing a deuterium atom to occupy a hexahedral interstice coordinated by three titanium atoms and two iron atoms. Forn −-2, i.e. for theγ phase, geometric considerations allow the correct prediction of some hydrogen atoms situated in octahedral interstices coordinated by four iron atoms and two titanium atoms. Occupation of such a site could not have been predicted by considering the relative affinities of the metals titanium and iron for hydrogen.     

Thermal stability of transition phases in zirconia-doped alumina

Alumina was prepared from an aqueous salt solution by homogeneous precipitation followed by calcination in air. Dependence of the thermal stability of transition phases on the presence of a zirconia dopant and on autoclave treatment prior to calcination was investigated using X-ray diffraction (XRD), differential thermal analysis coupled with thermogravimetric analysis (DTA–TGA) and transmission electron microscope (TEM) analysis. Homogeneous precipitation produced an amorphous trihydrate precipitate; the autoclave treatment converted this to crystalline boehmite (monohydrate). The zirconia was soluble in the transition alumina but was insoluble in α-Al₂O₃ so that phase transformation to α-Al₂O₃ was accompanied by a phase separation to form an alumina-zirconia nanocomposite. The thermal stability of the transition phases was increased both by the dopant and by the autoclave treatment. A combination of both parameters yielded the most stable transition alumina, which withstood 1 h at 1200°C without transformation to α-Al₂O₃. This revised version was published online in November 2006 with corrections to the Cover Date.     

Systematics in the electron transport and magnetic properties of LnBO3 perovskites

Systematics in the electrical and magnetic properties of transition metal perovskites LnBO₃ (Ln=rare-earth ion, B=3d transition metal) with the variation of Ln and B ions are reviewed. The electrical resistivity and activation energy of LnBO₃ compounds increase with the decreasing size of the Ln³⁺ ion for a given B ion. The low-spin to high-spin transition temperature of Co³⁺ ion in LnCoO₃ similarly increases with the decrease in size of Ln³⁺ while the magnetic ordering temperatures in LnVO₃, LnFeO₃, LnCrO₃ and LnSrCo₂O₆ decreases with decreasing size of the rare-earth ion. These results may be understood in terms of the increasing acidity of the rare earth ion with decreasing size and the competition between the Ln³⁺ and the B³⁺ ions for covalency with the oxygen ions. The effect of this competition on the metal oxygen covalency and crystal field parameter is discussed in relation to the results obtained and Goodenough’s phase diagram. The possibility ofd-f exchange interaction in La_1−xLn_x NiO₃ is also discussed in the light of ESR results. Communication No. 66 from the Solid State and Structural Chemistry Unit.     

Which tool to distinguish transient alumina from alpha alumina in thermally grown alumina scales?

Abstract

Alumina scales constitute excellent protective barriers when they form on alumina-forming steels. If they keep tightly adherent to the underlying substrate, they isolate it from the surrounding aggressive atmosphere at high temperature. The protectiveness of the alumina scale is highly dependant upon its growth mechanism. The nucleation and transformation of transient alumina (mainly γ-Al₂O₃ and θ-Al₂O₃) is known to play an important role on alumina scale formation. It is therefore fundamental to characterise these transient alumina especially during the early stages of the oxidation process. The morphology of the transient alumina was observed by scanning electron microscopy (SEM), their crystallographic phases determined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). X-ray photoelectron spectrometry (XPS) analyses were performed on reference samples and then compared to the alloys oxidised 5 and 30 minutes at 850, 900 and 950°C.     

Phase Transition of Superfluid 3He in Aerogel

We have studied phase transition of superfluid ³He in 97.5% porosity aerogel by NMR method. Above 1.0 MPa, superfluid phase transition has been observed. The transition temperature T _c ^a is strongly suppressed from its bulk value. The Pressure-Temperature diagram suggests that superfluid phase will not appear below near 0.8 MPa. The A-B phase transition has been observed above 1.3 MPa, below which a state of superfluid phases remains to be identified. The temperature dependence of NMR frequency shifts Δf in the A-like and the B-like phases are almost linear at pressures below 2.4 MPa. We obtained the differential coefficient of NMR frequency shifts ∂(Δf)/∂(T/T _c ^a ) at 0.9T _c ^a as a function of pressure, and it suggests that superfluid phase will not appear below near 0.8 MPa which is the same pressure estimated by P-T diagram.     

Evidence for a threshold in the biosolubility of aluminosilicate vitreous fibers

Two series of aluminosilicate glasses have been synthesized with the nominal composition (64 − x) SiO₂–x Al₂O₃–36 Na₂O/CaO with x varying from 9 to 19 mol%. They have been corroded in static conditions in a solution that mimics in a simplified manner the intracellular medium of the lung alveolar macrophages (37 °C, pH 4.6, citric acid). The original and corroded glasses have been studied by ²⁷Al and ²⁹Si MAS NMR. Both series display a sharp increase in the silicon dissolution rate with the alumina content. The glass network dissolves extremely slowly, whereas the release of excess sodium is very fast, for the glasses with low alumina content. On the opposite, the glasses with high alumina content dissolve much more rapidly in a nearly congruent manner. The crossover between the two behaviors occurs for x = 13, which corresponds to 33% of aluminum in the glass-former network. The sharp crossover from slow to fast network dissolution is explained in terms of connectivity of the silica sub-network. Above a certain amount of alumina, the silicon sub-network is no more percolating and the corroded glass breaks up into colloids. The sharpness of the transition and the relatively low alumina content required for fast dissolution are related to a structural feature of the aluminosilicate glasses, namely the aluminum self-avoidance that decreases the connectivity of the silica sub-lattice.     

A Spin Resonance Investigation of Magnetism and Dynamics in the Charge-transfer Salts β"-(BEDT-TTF)4[(H3O)M(C2O4)3]S

We report a spin resonance study of the family of quasi-two-dimensional organic (super)conductors β”-(BEDT-TTF)₄[(H₃O)M(C₂O₄)₃]S, where M is a 3d transition metal ion and S is a host solvent molecule. The spin systems for M = Cr³⁺ (S = 3/2) and M = Fe³⁺ (S = 5/2) are investigated by means of both resonant and field modulation techniques in the frequency range between 50 and 313 GHz. The role of the different solvent molecules in determining the degree of spin-orbit coupling and the local symmetry at the metal ion site is established. The low temperature behaviour of intensities, positions and widths of the resonant lines shows significant modifications of the spin-orbit coupling, and of the inter-and intra-ionic spin-spin inter actions. Despite the onset of a weak antiferromagnetic internal field at low temperature, the ultimate narrowing of the lines suggests spin-lattice interactions may still be the dominant relaxation process. Diamagnetic screening in the mixed state of the superconducting samples for fields parallel to the quasi-two-dimensional layers induces additional lineshifts only below B = 2.5T and T = 4K, determining the threshold of full field penetration within the anion layers.     

Atomic Insights into Phase Evolution in Ternary Transition‐Metal Dichalcogenides Nanostructures

Phase engineering through chemical modification can significantly alter the properties of transition‐metal dichalcogenides, and allow the design of many novel electronic, photonic, and optoelectronics devices. The atomic‐scale mechanism underlying such phase engineering is still intensively investigated but elusive. Here, advanced electron microscopy, combined with density functional theory calculations, is used to understand the phase evolution (hexagonal 2H→monoclinic T′→orthorhombic T_d) in chemical vapor deposition grown Mo_{1− x} W _x Te₂ nanostructures. Atomic‐resolution imaging and electron diffraction indicate that Mo_{1− x} W _x Te₂ nanostructures have two phases: the pure monoclinic phase in low W‐concentrated (0 < x ≤ 10 at.%) samples, and the dual phase of the monoclinic and orthorhombic in high W‐concentrated (10 < x < 90 at.%) samples. Such phase coexistence exists with coherent interfaces, mediated by a newly uncovered orthorhombic phase T_d′. T_d′, preserves the centrosymmetry of T′ and provides the possible phase transition path for T′→T_d with low energy state. This work enriches the atomic‐scale understanding of phase evolution and coexistence in multinary compounds, and paves the way for device applications of new transition‐metal dichalcogenides phases and heterostructures.     

Material Effect and Steam Explosion at High Temperature (<Emphasis Type="Italic">T</Emphasis> > <Emphasis Type="Bold">2300</Emphasis> K)

In the frame of nuclear power plant safety, the interaction of molten corium (mixture of materials coming from a power plant) with water can generate dynamic loading of the surrounding structures. This phenomenon is called the steam explosion. Many experiments have been performed in the KROTOS facility with simulation materials (Al₂O₃) and prototypical materials (U,Zr)O₂, and different behaviors attributed to a ‘material effect’ have been observed. Alumina melts produced spontaneous energetic steam explosions, whereas explosions with corium melts (80% UO₂–20% ZrO₂) must be triggered and are less energetic. These differences may be partly attributed to the formation of meta-stable gamma alumina and the ability of liquid alumina to dissolve part of the water, acting like an internal trigger. These results mean that alumina is probably not an adequate simulation of the corium for steam explosion. Paper presented at the Seventh International Workshop on Subsecond Thermophysics, October 6–8, 2004, Orléans, France.     

The growth and electrical transport properties of self-organized metal/oxide nanostructures formed by anodizing Ta-Al thin-film bilayers

Anodizing of Ta-Al metal bilayers (Al on Ta) sputter-deposited onto SiO₂ substrates was performed in oxalic acid electrolytes at anode potentials of 53 to 21.5 V in order to form nanoporous alumina layers and sequentially oxidize the tantalum underlayers through the alumina pores. The films formed consist of arrays of tantalum oxide nanohillocks percolating through the residual tantalum layer down to the substrate, so that a self-organized network of tantalum nanowires forms between the substrate and the alumina film. The average width (25–<10 nm), length (70–35 nm), and population density (10⁹–10¹¹ cm^-2) of the nanowires are systematically defined by the initial tantalum thickness (8–22 nm) and the anodizing conditions. The mesh-like, nano-sized morphologies of the tantalum underlayers result in a remarkably wide range of potential-dependent, controlled electrical sheet resistances (10²–10⁷ _Ω/sq). The periodical, tunable, metal/insulator film structure, allowing an increased transition to hopping or tunneling conduction at elevated temperature, leads to negative temperature coefficients of resistance, ranging 300 to 5 ppm/K. Oscillations of the potential-dependent dc conductance registered in the films at room temperature are attributed to the quantum-size effects in the metal/oxide nanostructures. The films are of technological importance for fabrication of thin-film, planar, adjustable resistors with significantly improved performances.     

Influence of ZrO<Subscript>2</Subscript> addition on the structure,thermal stability,and dielectric properties of ZnTiO<Subscript>3</Subscript> ceramics

The zinc titanates doped with zirconium were synthesized by conventional solid-state reaction using metal oxides. X-ray diffractometry and differential scanning calorimetry analysis results indicated that the stable region of the hexagonal Zn(Zr _x Ti_1−x )O₃ phase extended to a high temperature (above 945 °C). The c/a value decreased as the Zr concentrations increased, which may be caused by the Zr⁴⁺ addition resulting in a shorter distance between the center ion and its nearest neighbors of the octahedron, and the bonding force between the B-site ion and oxygen ion of ABO₃ perovskite-like structure becoming stronger. The dielectric properties exhibited a significant dependence on the sintering temperatures and the amount of ZrO₂ addition. The dielectric constant decreased and Curie temperature (T _c) increased slightly with the increasing amounts of Zr ions. This is caused by the second phase of ZnZrO₃ which was deposited at the grain boundaries and inhibited the grain growth. Furthermore, diffuse phase transition with a maximum permittivity at a transition temperature that is close to room temperature in Zn(Zr _x Ti_1−x )O₃ was observed.     

Synthesis of a high-boron aluminum boride via borothermic reduction of alumina

The reaction between alumina and boron in vacuum was studied between 800 and 1500‡C at different compositions of the starting mixture. The reaction products were analyzed chemically and by x-ray diffraction, optical, and microstructural analysis techniques. The compositions of the reaction intermediates and products were determined, which made it possible to propose a reaction scheme which differs significantly from the commonly accepted one and assumes that not only B₂O₃ but also AlO are removed in the course of the reaction. It is shown that only 60% of the starting Al is incorporated into the forming phase, AlB_18-31 (Β-B hexagonal structure,a = 1.0970 nm,c = 2.3780 nm). No aluminum dior dodecaboride was detected in the reaction products. The proposed mechanism of the process explains why the borothermic reduction of various metal oxides typically yields only the highest boron phases.     

Preparation and studies of some thermal,mechanical and optical properties of<Emphasis Type="Italic">x</Emphasis>Al<Subscript>2</Subscript>O<Subscript>3</Subscript>(1 −<Emphasis Type="Italic">x</Emphasis>)NaPO<Subscript>3</Subscript> glass system

Sodium aluminophosphate glasses having compositions of xAl₂O₃(1-x)NaPO3 (x = 0.05-0.2) were prepared using conventional melt-quench technique. Density, glass transition temperature, microhardness (MH), thermal expansion coefficient (TEC) and transmission characteristics were measured as a function of alumina content for different samples. They were found to depend on O/P ratio with pronounced changes taking place for O/P ratio ≥3.5. Density, glass transition temperature and microhardness were found to increase up to 15 mol% of alumina and then they showed a decreasing trend. Thermal expansion coefficient decreased continuously with alumina content. Optical gaps for different glass samples as measured from transmission characteristics were found to be in the range 3.13–3.51 eV. It initially decreased with alumina content up to 15 mol% and then increased. The behaviour was explained on the basis of change in the average aluminum coordination number from six Al(6) to four Al(4) (i.e. Al(OP)₆/Al(OP)₄ ratio) along with the changes in polyhedra linkages in the glass network due to change in O/P ratio.     

Theoretical Study of Specific Heat, Density of States and Free Energy of Itinerant Ferromagnetic Superconductor URhGe

Following the equation of motion method and Green’s function technique, the coexistence of itinerant ferromagnetism (FM) and superconductivity (SC) is investigated in a single band homogeneous system. Self-consistent equations for superconducting order parameter (Δ) and magnetic order parameter (Δ_FM) are derived. It is shown that there generally exists a coexistent (Δ≠0 and Δ_FM≠0) solutions to the coupled equations of the order parameter in the temperature range 0<T<min (T _C,T _FM) where T _C and T _FM are respectively the superconducting and ferromagnetic transition temperatures. Expressions for the specific heat, density of states and free energy are derived. The specific heat has a linear temperature dependence at low temperatures as opposed to the exponential decrease in the BCS theory. The density of states for a finite Δ_FM increases as opposed to that of a standard ferromagnetic metal. The free energy shows that the superconducting ferromagnetic state has lower energy than the normal ferromagnetic state and therefore is realized at low enough temperature. The theory is applied to explain the observations of URhGe. The agreement between theory and experimental results is quite satisfactory.      

Analysis on insulator–metal transition in yttrium doped LSMO from electron density distribution

Yttrium doped LSMO (La_1 − x Sr _x MnO₃) was prepared using sol–gel technique and analysed for the insulator–metal transition from charge density variation in the unit cell with respect to different stoichiometric inclusion of yttrium. X-ray powder diffraction profiles of the samples were obtained and the well known Rietveld method and a versatile tool called maximum entropy method (MEM) were used for structural and profile refinement. The charge density in the unit cell was constructed using refined structure factors and was analysed. The charge ordering taking place in the insulator–metal transition was investigated and quantified. The insulator–metal transition was found to occur when 20% of La/Sr atoms were replaced by yttrium. The changes in the charge environment have also been analysed.     

Magnetic-Field Induced Superconductor–Antiferromagnet Transition in Lightly Doped RBa2Cu3O6+x (R = Lu, Y) Crystals

The remarkable sensitivity of the c-axis resistivity and magnetoresistance in cuprates to the spin ordering is used to clarify the doping-induced transformation from an antiferromagnetic (AF) insulator to a superconducting (SC) metal in RBa₂Cu₃O_6+x (R = Lu, Y) single crystals. The established phase diagram demonstrates that the AF and SC regions apparently overlap: The superconductivity in RBa₂Cu₃O_6+x , in contrast to La_2−x Sr _x CuO₄, sets in before the long-range AF order is completely destroyed by hole doping. Magnetoresistance measurements of superconducting crystals with low T _c ≤15–20 K give a clear view of the magnetic-field induced superconductivity suppression and recovery of the long-range AF state. What still remains to be understood is whether the AF order actually persists in the SC state or just revives when the superconductivity is suppressed, and in the former case, whether the antiferromagnetism and superconductivity reside in nanoscopically separated phases or coexist on an atomic scale.     

Coexistence of Superconductivity and Itinerant Ferromagnetism in Ferromagnetic Metals UCoGe and UIr

A microscopic coexistence of itinerant ferromagnetism (FM) and superconductivity (SC) is studied in a single band homogenous system, following an equation of motion method and Green’s function technique. Self-consistent equations for superconducting order parameter (Δ) and magnetization parameter (M) are derived. It is shown that there generally exists a coexistent (Δ≠0, M≠0) solution to the coupled equations of the order parameters in the temperature range 0<T<min (T _C,T _FM), where T _C and T _FM are respectively the superconducting and ferromagnetic transition temperatures. The expressions for electronic specific heat (C/T), density of states, free energy, transition probabilities, ultrasonic attenuation, and nuclear relaxation are also derived. The theory is applied to explain the observations in UCoGe and UIr. The specific heat capacity at low temperature shows linear temperature dependence as opposed to the activated behavior. Density of states increases as opposed to the case of a standard ferromagnetic metal. Free energy study reveals that the superconducting ferromagnetic state has lower energy than the normal ferromagnetic state and, therefore, coexistence of FM and SC realized at a low enough temperature. The agreement between theory and experimental results for UCoGe and UIr is quite encouraging.