Synthesis and photoluminescence properties of nanostructured mullite/α- Al2O3Al_2O_3

Phase transformation of α-Al2O3 to mullite was observed by correlated X-ray diffraction and low-temperature (7 K) time- and energy-resolved photoluminescence methods. The aluminosilicate solids with different Si:Al ratios were prepared after thermal treatment (1400 °C) of ultraporous alumina (UPA) monoliths impregnated with vapour of silica precursors at room temperature. The initial dispersion of Si is homogeneous on a size-scale of 5 nm raw UPA fibres, which grow in size during the thermal treatment stage up to ∼50–100 nm. Three phases were resolved in the considered SiO2/Al2O3 system: silica, α-Al2O3 and their interaction product 2:1 mullite (2Al2O3:SiO2). The 2:1 mullite mass increases and α-Al2O3 mass decreases with an increase of the initial silica content. At the SiO2 content in the initial system above 20 mol.%, the phase transformation to 2:1 mullite is complete and no α-Al2O3 was observed. No mullite phases with either lower or higher Si content were observed. The fundamental absorption onset energy 7.55 eV of 2:1 mullite was measured

Heteroepitaxial growth of ZrO

We have studied Zr1−XCeXO2 film growth on (001) Si by Pulsed Laser Deposition using sputtering of metallic alloy targets and sintered ceramic targets. The conditions of the epitaxial growth have been found and optimized. The epitaxial oxide film growth (001) [100]||(001) [100] Si was obtained for a range of CeO2 content in ZrO2: from 4.5 up to 14% mol in ZrO2. The oxide film structure corresponds to a tetragonal phase with strong preference for c-axis orientation normal to the growth surface. The results obtained by RHEED, XRD and AFM methods have confirmed the high quality of heteroepitaxial Zr1−XCeXO2 layers, and the difference in crystallinities for the films grown from metallic alloy targets and ceramic targets was evaluated. The XRD results show the absence of any reflection distinct from (00l) and (l00) for films grown from alloy targets, and, in contrast with this, the film structure contains some random oriented inclusions in the case of oxide target deposition. Use of metallic alloy Zr-12% Ce targets and low oxygen pressure during deposition provide the best film quality with the minimum of surface microrelief (Rrms < 0.3 nm for 1 × 1 µm2 surface area was achieved)

Porous monoliths consisting of aluminum oxyhydroxide nanofibrils: 3D structure, chemical composition, and phase transformations in the temperature range 25–1700 °C

We present a study on the chemical and structural transformations in highly porous monolitic materials consisting of the nanofibrils of aluminum oxyhydroxides (NOA, Al2O3·nH2O) in the temperature range 20–1700 °C. A remarkable property of the NOA material is the preservation of the monolithic state during annealing over the entire temperature range, although the density of the monolith increases from ~0.02 up to ~3 g/cm3, the total porosity decreases from 99.3 to 25% and remains open up to 4 h annealing at the temperature ~1300 °C. The physical parameters of NOA monoliths such as density, porosity, specific area were studied and a simple physical model describing these parameters as the function of the average size of NOA fibrils—the basic element of 3D structure—was proposed. The observed thermally induced changes in composition and structure of NOA were successfully described and two mechanisms of mass transport in NOA materials were revealed. (i) At moderate temperatures (T ≤ 800 °C), the mass transport occurs along a surface of amorphous single fibril, which results in a weak decrease of the length-to-diameter aspect ratio from the initial value ~24 till ~20; the corresponding NOA porosity change is also small: from initial ~99.5 to 98.5%. (ii) At high temperatures (T > 800 °C), the mass transport occurs in the volume of fibrils, that results in changes of fibrils shape to elliptical and strong decrease of the aspect ratio down to ≤ 2; the porosity of NOA decreases to 25%. These two regimes are characterized by activation energies of 28 and 61 kJ/mol respectively, and the transition temperature corresponds to the beginning of γ-phase crystallization at 870 °C

Superhydrophobic and luminescent highly porous nanostructured alumina monoliths modified with tris(8-hydroxyquinolinato)aluminium

A straightforward and facile procedure for the fabrication of superhydrophobic luminescent 3D nanomaterials was developed. Chemical modification of ultra-lightweight highly porous nanostructured aluminum oxyhydroxide (NOA) monoliths in 8-hydroxyquinoline vapors resulted in the formation of tris(8–hydroxyquinoline)aluminum on the surface of NOA nanofibrils. The original shape and size of the initial NOA monolith and its internal 3D nanostructure were completely preserved during the modification. Surface modified NOA samples demonstrated intense green luminescence as well as superhydrophobicity, the water contact angle being ~153°, the sliding angle ~6° and contact angle hysteresis ~8°. We believe that an unusual combination of properties inherent in the synthesized material will be advantageous for the design of water-proof self-cleaning photonic devices

Structural Analysis of Aluminum Oxyhydroxide Aerogel by Small Angle X-Ray Scattering

The work presents studies on the microstructure and mesostructure of nanostructured aluminum oxyhydroxide formed as a high porous monolithic material through the surface oxidation of aluminum liquidmetal solution in mercury in a temperature- and humidity-controlled air atmosphere. The methods of X-ray diffraction analysis, thermal analysis, the low temperature adsorption of nitrogen vapors, transmission electron microscopy, small-angle and very small-angle neutron scattering, and small-angle X-ray scattering are used for comprehensive investigation of the samples synthesized at 25°С as well as that annealed at temperatures up to 1150°C. It is found that the structure of the monolithic samples can be described within the framework of a three-level model involving primary heterogeneities (typical length scale of rc ≈ 9–19 Å), forming fibrils (cross-sectional radius R ≈ 36–43 Å and length L ≈ 3200–3300 Å) or lamellae (thickness T ≈ 110 Å and width W ≈ 3050 Å) which, in turn, are integrated into large-scale aggregates (typical size R c ≈ 1.25–1.4 μm) with an insignificant surface roughness. It is shown that a high specific surface (~200 m2/g) typical for the initial sample is maintained upon its thermal annealing up to 900°С, and it decreases to 100 m2/g after heat treatment at 1150°С due to fibrillary agglomeration