Dielectric relaxation study of hydrogen exposure as a source of two-level systems in Al2O3

An important question in the manufacture of superconducting electronics is how to control the two-level systems found in amorphous insulators. The present article shows that hydrogen has a marked impact on the two-level systems in thin films of reactively sputtered Al₂O₃, a standard tunnel oxide for Josephson junctions. The magnitude of dielectric relaxation current in Al₂O₃ films, believed to be caused by two-level systems, is shown to increase monotonically with the flow rate of H₂ into the chamber during deposition. This points toward a potential need for controlling hydrogen during the manufacture of superconducting electronics utilizing Al₂O₃.     

Semi-quantitative analysis for FTIR spectra of Al2O3-PbO-B2O3-SiO2 glasses

A series of glass of the molar formula xAl₂O₃-(50-x)PbO-25B₂O₃-25SiO₂ with x:2.5–17.5 with step of 2.5 mol.% was prepared and measured for, density, molar volume and infrared absorption. A semiquantitative analysis of the IR spectra was performed. It was found that each oxide would contribute in density with a specific factor. The density factor related to PbO is markedly higher than that of the other two oxides represent the glass skeleton which means that the content of PbO is the main factor affecting the density. The depolymerization of the whole glass skeleton increases with increasing the content of Al₂O₃. There is a competition between the role of PbO and Al₂O₃ in changing the value of N₄ and the crosslinking of the glass network. The silicate network tends to be distinguished from the reminder of the whole glass network with increasing alumina. The IR band located near 700 cm^? 1 was suggested to be due to the vibrations of bridging oxygens between trigonal boron atoms. An essential change in the role of PbO in these glasses from glass modifier to glass former occurred around 12 mol.% Al₂O₃.     

Thermal expansion and transition temperature of glasses in the systems BeO–Al2O3–B2O3 and MgO–Al2O3–B2O3

Researches were conducted for glasses in the systems BeO–Al₂O₃–B₂O₃ and MgO–Al₂O₃–B₂O. The following characteristics have been determined: thermal expansion coefficient within 20–300 °C, structural thermal expansion coefficient (STEC) and glass transition temperature. The discussed properties of Be-aluminoborate and Mg-aluminoborate glasses have been compared with those of Ca-, Sr- and Ba-aluminoborate glasses. TEC of studied glasses gets higher going from Be-aluminoborate glasses to Ba-aluminoborate ones, but T_g decreases under the same succession. The dependence of STEC on the nature of a given cation is more complicated. The pattern of dependence of the properties on composition is due to changing the boron coordination number with respect to oxygen, which is the main network-former, and to competition between the aluminium and boron for the oxygen brought by an ion-modifier. The difference between the effect of one ion-modifier and that of another is determined by decreasing the radius of an ion and increasing its electric field strength in the succession from Ba²⁺ to Be²⁺. The ions of Be and Mg can also act as network-former to some extent.     

Effects of yttrium codoping on fluorescence lifetimes of Er3+ ions in SiO2–Al2O3 sol–gel glasses

In order to find a new glass host and optimize erbium doping for IR glass optical amplifiers in photonic applications, a study on the optimization of the emission of erbium ions in the SiO₂–Al₂O₃ glass by codoping with Y₂O₃ is performed. It is first attempted to make a new sol–gel glass host based on SiO₂, Al₂O₃, and Y₂O₃ doped with Er³⁺ ions of the composition (1−x)SiO₂–xAl₂O₃–yY₂O₃:0.65Er₂O₃ (in mol%), x varies from 0 to 65, and y from 0 to 4. The optimal proportion in mol% of SiO₂ and Al₂O₃ for the Er³⁺ emission (at a fixed optimal concentration of 0.65) was 65 – 35. The effect of Y₂O₃ content on photoluminescence, decay curve profiles and lifetime of the ⁴I_13/2 level of Er³⁺ in SiO₂–Al₂O₃ glass is observed. The largest quantum efficiency and the higher emission intensity are observed in the sample with 65Al₂O₃ and 4Y₂O₃. The emission intensity at 1530 nm is two times higher than in glasses without Y₂O₃. A shift of 3 nm to shorter wavelengths is observed. The emission spectral profiles are flatter and broader for the glasses containing Al and Y (bandwidth of 59.5 nm). The decay curves show strong difference profiles for the different samples. The increase of the lifetime value τ (about ms) of the ⁴I_13/2 level of Er³⁺ in the SiO₂–Al₂O₃ with the Y₂O₃ is discussed.     

Oxidation–reduction processes upon interaction of aluminum oxide melt with molybdenum and tungsten in a hydrogen-containing atmosphere

A thermodynamic analysis of the processes occurring in the Мо–W–Al₂O₃ system at T = 2400 K and a total pressure of 1 bar, set by controlled reducing Ar + H₂ atmosphere, has been performed. It is found that the basic components of the system do not interact directly, although may be actively involved in chemical reactions with participation of other components to undergo numerous cyclic oxidation–reduction processes. Particular attention is paid to the processes involving such chemically active reagents as Н₂O₂, HO₂, H₂ (H), gaseous Al, and its hydrides (AlH, AlH₂, AlH₃).

     

Some Aspects of Interactions in the Mo–W–Al2O3–H2 System: Formation of Polyoxides

The possibility of formation of molybdenum and tungsten polyoxides in the Mo–W–Al₂O₃–H₂ system at T = 2400 K and P = 1 bar in a controlled Ar + H₂ atmosphere has been investigated by the method of thermodynamic analysis. The formation of polyoxides is found to occur both due to the processes involving Al₂O₃ melt and in the absence of the latter. It is established that metals (Mo and W) and their mono-, di-, and even trioxides (in the latter case, mediated polymerization occurs) can be used as initial components to form polyoxides. It is shown that polyoxides themselves may interact with one of their main sources: Al₂O₃ melt.

     

Purity and doping possibilities of Al2O3 and YAG molten in Mo crucibles and crystals grown from this melt

Doping possibilities of Al₂O₃ and YAG crystals grown from the melt alternatively doped with alkali earth, silicon, iron group or molybdenum ions under 98% Ar – 2% H₂ or 98% He – 2% H₂ protective atmosphere are described. Alkali earth and particularly Si ions evaporate slowly from the melt. Reduction of iron group ions was observed. Mo may enter YAG phase using a wet protective atmosphere. Al₂O₃ phase contains Mo ions if grown from the electrolyzed melt.     

Ionic liquid‐assisted hydrothermal synthesis of γ‐Al2O3hierarchical nanostructures

The boehmite (Al₂O₃·H₂O) hierarchical nanostructure with spindle‐like morphology has been successfully synthesized via ionic liquid‐assisted hydrothermal synthetic method under mild condition using an ionic liquid 1‐butyl‐3‐dimethylimidazolium bromide ([Bmim][Br]) as a template. The proposed formation mechanism has been investigated and the hydrogen bond‐co‐π–π stack mechanism is used to be responsible for the present formation of the precursor hierarchical nanostructure. The γ‐Al₂O₃ hierarchical nanostructure was obtained by calcining the as‐synthesized precursor at 500 °C for 2 h, preserving the same morphology. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermal expansion of glass–ceramics in the system BaO/Al2O3/B2O3

Glasses in the system BaO/Al₂O₃/B₂O₃ with and without the addition of platinum were melted. In one sample series, the BaO-concentration was varied while the ratio [Al₂O₃]/[B₂O₃] was kept constant. In another sample series, the [BaO]/[Al₂O₃]-ratio (= 0.9) was kept constant and the B₂O₃ concentration was varied. The samples were thermally treated at 720 °C for 24 h and subsequently at 780 °C for 4 h. In most thermally treated samples, the crystalline phase BaO · Al₂O₃ · B₂O₃ occurred. At some compositions, the platinum-doped samples showed larger concentrations of the crystalline phases. The most remarkable property of the obtained glass–ceramics is their zero or negative thermal expansion coefficient. Here, notable differences were observed: samples with fine grained microstructures showed thermal expansion coefficients approximately zero up to temperatures of around 80 °C. By contrast, samples with coarser microstructures and large spheroidal crystals exhibit negative expansion coefficients up to temperatures of around 280–375 °C. The thermal expansions of these samples were close to those of the mean thermal expansion of the unit cell of the BaO · Al₂O₃ · B₂O₃ phase. The thermal expansion of the fine grained samples was approximately equal to that of the crystallographic a-axis of the BaO · Al₂O₃ · B₂O₃ phase.     

Mechanisms of Interaction of Molybdenum and Tungsten Polyoxide with Aluminum Oxide Melt under Reducing Conditions

The main chemical reactions between Mo and W polyoxides and Al₂O₃ melt in a controlled Ar + H₂ atmosphere (T = 2400 K, P = 1 bar) during sapphire growth by horizontal directional solidification have been investigated. Under these thermodynamic conditions, the melt and products of its dissociative evaporation may actively react with the tungsten heater and molybdenum thermal screens of the crystallization system. It is shown that the polyoxides formed during evaporation do not directly interact with the melt; this interaction occurs only with participation of reagents exhibiting pronounced reducing properties (Al, H₂, H, WO, Al₂O, AlH, AlH₂, AlH₃). It is established that most of processes occur with participation of aluminum hydrides. A particular role of Mo(W) dioxide–W(Mo) polyoxide functional pairs in the interaction with the melt is determined.

     

Production of Y3Al5O12 and Y2O3 nanopowders for optical ceramics

The physical and technical conditions for reproducible production of nanodispersed yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) and yttrium oxide (Y₂O₃) powders by chemical coprecipitation have been investigated. It is established that the obtained YAG nanopowders have enhanced reactivity, which significantly decreases the temperature range of interaction in the Y₂O₃-Al₂O₃ system in comparison with ceramic synthesis. It is shown that vacuum heat treatment may lead to reversible transformation of the YAG crystal structure from cubic to tetragonal.     

Direct and outer-sphere coordination of the magnesium ions in the crystal structures of complexes with 2-furancarboxylic acid (I) and 3-furancarboxylic acid (II)

(I) Mg₂C₂₀H₂₄O₁₈ monoclinic,PT,a=10.760(2) Å,b=11.052(2) Å,c=12.822(3) Å, α=105.31(3)^o, β=98.18(3)^o, γ=91.59(3)^o,Z=2. (II) MgC₁₀H₁₄O₁₀, monoclinic,C2/c,a=30.817(6)Å,b=10.499(2)Å,c=9.000(2)Å, β=91.31(3)^o,Z=8. Magnesium in complexes with furoic acids reveals two ways of coordination: direct, when furoic anions are bonded to Mg²⁺ in an ionic fashion and outer-sphere, when cations bind water in the first coordination sphere and furancarboxylic ligands are hydrogen bonded to the water molecules. This results in the formation of three bridging systems: ?Mg?O_carboxyl?C?O_carboxyl?Mg?, ?Mg?O_water ?O_carboxyl?C?O_carboxyl?C?O_carboxyl?Mg?, and ?Mg?O_water?O_carboxyl?C?O_carboxyl?O_water?Mg?. Magnesium 2-furancarboxylate (I) is dimeric, while magnesium 3-furancarboxylate (II) exhibits a polymeric structure.     

Crystal structure and IR spectroscopic study of (CN3H6)2[(UO2)2(C2O4)(CH3COO)4]

Compound (CN₃H₆)₂[(UO₂)₂(C₂O₄)(CH₃COO)₄] is synthesized and characterized by IR spectroscopy and single-crystal X-ray diffraction [a = 8.5264(2) Å, b = 13.8438(4) Å, c = 10.7284(2) Å, β = 103.543(1)°, space group P2₁/n, Z = 2, and R = 0.0258]. The main structural units of the crystals are binuclear [(UO₂)₂C₂O₄(CH₃COO)₄]^2? groups, which belong to the A ₂ K ⁰² B ₄ ⁰¹ crystal chemical group of uranyl complexes (A = UO ₂ ²⁺ , K ⁰² = C₂O ₄ ^2? , and B ⁰¹ = CH₃COO^?). The coordination polyhedron of the uranium atom is the UO₈ hexagonal bipyramid with the oxygen atoms of the uranyl ion at the axial positions. Uranium-containing groups and guanidinium cations are connected by electrostatic interactions and by the hydrogen bond system, which involves hydrogen atoms of guanidinium cations and oxygen atoms of oxalate and acetate anions. The results of the spectroscopic study of the compound agree with the X-ray diffraction data.     

Crystal structures and luminescence properties of novel compounds K4M2[Al2Si8O24] (M = Ce, Gd)

Two novel potassium rare earth silicates, obtained by hydrothermal synthesis, have been investigated by X-ray diffraction and described by the general formula K₄ M ₂[Al₂Si₈O₂₄] (M = Ce, Gd). The parameters of the monoclinic K₄Ce₂[Al₂Si₈O₂₄] and K₄Gd₂[Al₂Si₈O₂₄] cells are, respectively, as follows: a = 26.867(1), 26.6520(2) Å; b = 7.4150(2), 7.2854(1) Å; c = 14.910(1), 14.8182(1) Å; β = 123.52(1)°, 123.46(1)°; and sp. gr. P2₁/n. The structures are solved by the charge flipping method and refined in the anisotropic approximation of thermal vibrations for atoms to R = 5.2 and 2.5%, respectively. The compounds under study are crystallized into a new structural type, which is based on two-level [Al₂Si₈O₂₄] layers, combined into a three-dimensional framework by columns of edge-sharing seven-vertex REE polyhedra. Potassium atoms are located in the framework channels. The spectral luminescence characteristics are determined. The luminescence bands are typical of Gd³⁺ and Ce³⁺ ions. Upon excitation by light with λ _exc = 246 nm, a band due to the ² D → ⁸ F _5/2 transition with λ_max = 430 nm is observed in the spectrum of K₄Ce₂[Al₂Si₈O₂₄] and a band related to the ⁶ P _7/2 → ⁸ S _7/2 transition with λ_max = 311 nm is observed in the spectrum of K₄Gd₂[Al₂Si₈O₂₄].     

Synthesis and crystal structure of [Nd(H2O)2]2(C2O4)3

Some crystals of [Nd(H₂O)₂]₂(C₂O₄)₃ were synthesized hydrothermally by heating at 200°C for 8 days an aqueous suspension of neodymium oxalate decahydrate in presence of terephthallic acid and guanidinium carbonate. They crystallize in the orthorhombic system, space group P2₁2₁2₁, with a = 8.6702(7) Å, b = 9.558(2) Å, and c = 17.009(2) Å. The structure of this complex is built up by two independent neodymium atoms, three bischelating oxalate ligands, and four water molecules forming a rectangle building unit of 6-membered ring, [Ln(H₂O)₂(C₂O₄)]₆. The packing of these units leads to a layer parallel to the plane (001). However, the neodymium atoms of two neighbor layers share an edge of oxalato oxygen atoms thus giving a double-layer. The three dimensionality between these double-layers is insured by hydrogen bonds of water molecules which are bound to the neodymium atoms. There is no zeolitic water molecule. The two neodymium atoms are nine-coordinated. In both cases, the coordination polyhedron can be described as a distorted tricapped trigonal prism.     

Effect of Al2O3 on the viscosity and structure of calcium silicate-based melts containing Na2O and CaF2

The effect of Al₂O₃ on viscosity in the calcium silicate melt-based system containing Na₂O and CaF₂ was investigated and correlated with the melt structure using FTIR (Fourier transform infrared) spectroscopy, XPS (X-ray photoelectron spectroscopy), and Raman spectroscopy. Substituting SiO₂ with Al₂O₃ modified the dominant silicate network into a highly structured alumino-silicate structure with the aluminate structure being particularly prevalent at 20 mass% of Al₂O₃ and higher. As the melts become increasingly polymerized with higher Al₂O₃ content, the fraction of symmetric Al–O⁰ stretching vibrations significantly increased and the viscosity increased. XPS showed a decrease in the amount of non-bridged oxygen (O^?) but an increase in bridged oxygen (O⁰) and free oxygen (O^2?) with higher Al₂O₃. Although changes in the structure and viscosity with higher CaO/(SiO₂ + Al₂O₃) were not significant, the symmetric Al–O⁰ stretching in the [AlO₄]^5?-tetrahedral units decreased. The apparent activation energy for viscous flow varied from 118 to 190 kJ/mol.     

Some defects appearing in chromium doped Al2O3 monocrystals as a result of thermal treatment

Results of experiments related to thermal treatment of Al₂O₃ crystals doped by chromium (ruby) in various gaseous media: CO, H₂, vacuum and O₂ are given. The kinetics of the formation and destruction of dye centres during this process is discussed. Special attention is given to revealing the causes of the appearance of an absorption band in the region of 315 nm which is related to the internal stability of the crystal with respect to intensive light radiation. – An assumption is made on the nature of the absorption band. – A computation is made of the diffusion coefficient of dye centres related to 315 nm absorption band. This is derived as a function of chromium concentration.     

Investigation of plasma interaction with carbon impurities on SiO2- and Al2O3 surfaces by EPR-, Mass- and emission spectroscopy

Carbon impurities on highly disperse SiO₂ and Al₂O₃ are detected by their interaction with low pressure plasmas, with aid of mass-, emission- and epr-spectroscopic methods. The formation of surface defects on SiO₂ by plasma treatment is strongly influenced by these impurities. Some of these defects are caused by the impurities also. These effects depend on the time of plasma interaction and on plasma gas, where Ar, O₂, H₂ and CO plasmas are investigated. By H₂ plasma and the reaction products of the other mentioned above plasmas a partial hydrogenation of the carbon impurities is detected.     

Preparation of textured porous Al2O3 ceramics by slip casting in a strong magnetic field and its mechanical properties

The textured porous Al₂O₃ ceramics were prepared by slip casting in a strong magnetic field of 6 T and subsequently sintering. The c axis of Al₂O₃ grain was oriented parallel to the direction of the magnetic field and the textured porous microstructure with plate‐shape grains was formed. The porosity of textured porous Al₂O₃ ceramic was 30.37% and the relative density reached 66.29% when the sintering temperature is 1600°C. The textured porous Al₂O₃ green body showed the linear shrinkage anisotropy. The bending strength of the textured porous Al₂O₃ ceramics depended on the alignment direction of plate‐shape grains.     

Devitrification behavior and structure of Li2O-B2O3-Al2O3 composite gels from metal alkoxides

(30 − x/2)Li₂O·(70 − x/2)B₂O₃·xAl₂O₃(x = 0, 5 and 10) composite gels have been fabricated by the sol-gel method. LiOCH₃, B(OC₄H₉)₃, and Al(OC₄H₉)₃ were used as precursor for Li₂O, B₂O₃, and Al₂O₃, respectively. B(OC₄H₉)₃ and Al(OC₄H₉)₃ were hydrolyzed separately and then mixed. The crystallization behavior and structure of the gels upon thermal treatment temperatures between 150 and 550 °C are characterized on the basis of SEM, XRD and IR analyses. Xerogel with x = 0 exhibits non-crystal features, whereas crystalline phases are found in the xerogels with x = 5 and 10. The crystalline phases are not found with increasing heat treatment temperatures from 150 to 450 °C, but crystalline phases appear present at 550 °C. The xerogel with x = 0, subject to thermal treatment below 450 °C, is found to be still amorphous, and a 550 °C heat treatment leads its structure changing from glassy to crystalline.