Yuri D. Tretyakov

Recent progress in cryochemical synthesis of oxide materials

Recently conventional cryochemical processes have been complemented by cryoextraction, cryoprecipitation, cryoimpregnation and freeze casting techniques. Considerably more attention is being paid to colloidal solution processing and the application of sol-gel procedures like ion exchange treatment and hydrolysis of organometallic compounds. New preparation methods for nanopowders of various oxides and metals have been developed while further treatment of cryochemical powders using powder engineering techniques allows the powder grain size to vary from 0.2 to 5-6 µm. Cryochemical methods are also suitable for preparation of various porous media, like porous silica or cryogels with surface areas close to those of aerogels, and for the synthesis of HTSC powders for different applications. Fundamental and applied aspects of cryochemical processing are discussed.

Chemical routes for preparation of oxide high-temperature superconducting powders and precursors for superconductive ceramics, coatings and composites

The importance of chemical routes for the preparation of high-temperature superconducting (HTSC) powders is discussed as well as preparative peculiarities of their synthesis by coprecipitation, sol–gel, freeze-drying and spray pyrolysis techniques.

Lattice expansion and oxygen non-stoichiometry of nanocrystalline ceria

Generalized dependence of unit cell parameter and oxygen non-stoichiometry on the particle size of nanocrystalline ceria synthesized by a wide variety of solution-based methods was established.

Photonic Crystal Formed by the Imaginary Part of the Refractive Index

2010 WILEY-VCH Verlag Gmb Photonic crystals (PhCs) are widely studied photonic structures that provide unprecedented control over the propagation of light. The large majority of PhCs are formed by a periodic modulation of the refractive index, i.e., a modulation of the real part of the dielectric constant. Additional functionality can be created by including absorbing features into the structure, thus creating PhCs out of materials with complex dielectric index. This is particularly interesting when the photonic resonance created by the refractive index contrast overlaps with the absorption feature, hence creating a ‘‘resonantly absorbing PhC’’. An example of such a resonantly absorbing structure is that of a semiconductor saturable absorber mirror (SESAM) used in mode-locked lasers where quantum wells are incorporated into a multilayer Bragg stack. Similar structures referred to as Resonantly Absorbing Bragg Reflectors (RABR) have been used to demonstrate optical switching, optical storage, and nonlinear optical conversion. The examples above are based on Bragg mirrors or 1D photonic crystals. An extension to 2D and 3D structures has been proposed and demonstrated by backfilling the voids of a conventional photonic crystal with resonantly absorbing materials such as quantum dots and metal. Most of these structures are based on refractive index modulation with absorption providing additional features. In contrast, the structure we propose and demonstrate here, an extension of the 1D case approaches of Prineas et al. and Kozhekin et al., is formed exclusively by the absorbing feature, hence it is a true ‘‘imaginary refractive index’’ structure. A refractive index contrast naturally exists near the absorption feature, as required by the Kramers–Kronig relationship, but away from this feature, the refractive index contrast is practically zero. In order to demonstrate this effect experimentally, we created a template using holographic lithography with a diffractive optical element (DOE) (Fig. 1), which generates a 2D photonic lattice of SU-8 polymer disks (Fig. 2a). The disks are doped with a high concentration of the organic dye Rhodamine B (RhB) that has an absorption peak around 564 nm, so the absorption of the lattice is strongly dependent on wavelength. Subsequent filling of the voids with the same SU-8 polymer, but without dye doping, gives rise to an imaginary index photonic lattice (Fig. 2b). The diffraction pattern of the structure before back-filling is given by the dielectric modulation that exists for all wavelengths as in a conventional photonic lattice and is much like a rainbow (Fig. 3a). By contrast, only yellow/green light diffraction around 564 nm can be observed once the structure has been back-filled (Fig. 3b–d). The wavelength dependent diffraction clearly shows that the structure only acts as a PhC in the vicinity of the absorption window. Out of this window, the structure behaves as a uniform polymer layer. This new type of photonic crystal offers intriguing properties for further study in the field of saturable light absorption and emission control. The PhC may also be applied for optical switching and optical logic operation. The imaginary index structure can be treated as a 2D grating with wavelength-dependent phase and intensity modulation. According to diffraction theory, the intensity distribution I created by the grating is given by the Fourier transform of the transmission function of the grating t:

Chemical tuning of adsorption properties of titanate nanotubes

A conventional hydrothermal method widely used for the preparation of titania-based nanotubes still generates many unsolved questions. One of them is definitely connected with the influence of a posthydrothermal treatment of titania nanotubes on their micromorphology, structure, and adsorption characteristics. Here, it was analyzed systematically by a group of methods including nitrogen adsorption and temperature-programmed desorption of ammonia and carbon dioxide. It is proved that adsorption characteristics and the surface state of titania nanotubes correlate with a sodium content, since sodium ions act as Lewis acid sites and shield Ti4+ acid sites of the nanotubes. To obey a balance between chemical and heat treatments of the nanotubes to design their functional properties has been suggested.

Synthesis and properties of Hg0.7Pb0.3(BaSr)2Ca2Cu3Oz superconductors

Abstract We have synthesized Hg(Pb)-1223 with substitutions of Sr for Ba, i.e. Hg 0.7 Pb 0.3 Ba 2− x Sr x Ca 2 Cu 3 O z ( x = 0.0–2.0) by the ampoule method. The spray-drying method and thermal decomposition under vacuum were applied for the preparation of homogeneous precursors. Samples with T c = 110–128 K were obtained without oxygen annealing. We found that substitution of Ba by Sr remarkably relaxed the requirements on the precursors preparation and allowed their handling under ambient conditions.

An experimental study of copper self-diffusion in CuO, Y2Cu2O5 and YBa2Cu3O7–x by secondary neutral mass spectrometry

Copper self-diffusion has been studied in CuO, Y2Cu2O5 and YBa2Cu3O7–xceramics by the SNMS (secondary neutral mass spectrometry) technique in the temperature range 700–900 °C with the stable isotope63Cu used as a tracer. The lowest diffusion rate and the highest value of activation energy were found for the Y2Cu2O5 phase. Copper self-diffusion was also studied along the c axis of YBa2Cu3O7–xsingle crystals. It was shown that the anisotropy of Cu diffusion in YBa2Cu3O7–xis rather high. The diffusion rate in thec direction of single crystals is more than two orders of magnitude lower than that in ceramic samples.

SUPERCONDUCTIVE WHISKER-LIKE CRYSTAL-GROWTH IN BI-SR-CA-CU-O AND MORE COMPLICATED OXIDE SYSTEMS

High-quality fibrous crystals have been grown by prolonged heating of amorphous plates in an oxygen flow. The whisker growth rate was varied between 0.005 and 0.1 mm h–1. The crystals obtained at high temperature had frozen liquid droplets. The body of the whiskers had reasonably constant composition, which did not correspond to the droplet composition. The growth of the fibres occurred by liquid-phase transport of Ca and Cu along the crystal from the amorphous plate to the crystallization zone. This mechanism is termed growth via melt transport under a temperature gradient (MTTG). Tc, Jc and gap parameter, Δ, values for several whiskers were measured. The highest Jc value (2 × 105 A cm–2 at 77 K) and the highest Tc value were achieved in (2212 + 2223) interlayered fibres.

Growth of thin vanadia nanobelts with improved lithium storage capacity in hydrothermally aged vanadia gels

A process of hydrothermal ageing of vanadia gels yields 10–20 nm thick and 90–100 nm wide nanobelts exceeding 10 microns in length. The diminished thickness and networking of anisotropic nanobelts lead to lithium intercalation capacities exceeding 450–500 mA h g−1 at a C/25 rate. The observed morphology features depend essentially on preparation conditions and allow to assume that this particular route results in a suitable morphology of nanobelts via chemical bond rearrangement in the course of olation and oxolation in the aged bulk gel. “Unzipping” of the layered structure of the precursor gel into single-crystalline nanobelts, and optimization of post-hydrothermal processing resulted in nanomaterials with enhanced electrochemical characteristics, making vanadia gels a precursor of choice for simple preparation of new battery nanomaterials.

Synthesis of colloidal solutions of SrFe12O19 plate-like nanoparticles featuring extraordinary magnetic-field-dependent optical transmission

Hard-magnetic plate-like nanoparticles of SrFe12O19 were synthesised hydrothermally. The particles, fixed in a polymer, exhibited a coercivity in the range 0.9–1.8 kOe, and the saturation magnetization values reached 64 emu g−1. Electrostatically stabilized aqueous colloidal solutions of the plate-like particles with average thickness of 5–8 nm and average diameter of 30–80 nm were prepared. The nanoparticles readily oriented when applying a weak magnetic field as low as a few Oe, and due to the linear dichroism phenomena a strong change in the optical transmittance of the solution took place. The extinction coefficient ratio for the perpendicular and the parallel lights electric field vector relative to the magnetic field direction was found to be up to 4.5. The experimental results were compared with a theoretical model, in which the optical absorption of the nanoparticles was calculated from the polarizability of oblate ellipsoids.