A.V. Knotko

Preparation of ordered magnetic iron nanowires in the mesoporous silica matrix

Abstract We report the synthesis of magnetic nanocomposites using mesoporous silica as a host material. Iron nanoparticles were incorporated into the pores of mesoporous silica. During the synthesis, a hydrophobic metal complex, Fe(CO)5, was introduced into the hydrophobic part of the as-prepared mesoporous silica–surfactant composite. The suggested method results in the formation of iron nanowires inside the silica framework. Particles shape and size are in good agreement with the shape and size of the pores. Particles are uniform and well ordered in the silica matrix. Thus, mesoporous silica serves as nanoreactor for the formation of Fe-nanoparticles. The magnetic susceptibility measurements indicate superparamagnetic properties of all samples. This approach leads to functional materials with nanosized active elements in amorphous silica matrix, which could find application as high-density data storage devices.

Studies of New Order-Disorder Structural Transitions in Ln2M2O7 (Ln = Lu, Gd; M = Ti)

Phase formation processes at T = 350–1000°C in Lu(Gd)-Ti-O samples obtained by co-precipitation method are studied. Thermal processing of freeze dried co-precipitation products results in formation of Lu2Ti2O7 with fluorite structure at 650°C. The phase transition of fluorite to pyrochlore occurs at 750–800°C. At T > 800°C Lu2Ti2O7 possesses the structure of the disordered pyrochlore with antistructural defects. Further heating leads to the ordering of pyrochlore at T > 900°C. This process is accompanied by the appearance of a maximum at the temperature dependence of dielectric permeability of Lu2Ti2O7. The grain size of Lu2Ti2O7 ceramics annealed at 950°C was found to be 13–15 nm. The electric conductivity of this ceramics at 200°C (10−5 S/cm) is 5 orders higher than the values usually observed for Lu2Ti2O7 fabricated by a common solid state synthesis method. In Gd-Ti-O system only pyrochlore crystallization process at 740–900°C was observed.

STRUCTURE AND ELECTRICAL CONDUCTIVITY OF LN2+XHF2-XO7-X/2 (LN = SM-TB; X = 0, 0.096)

Data are presented on the evolution of the pyrochlore structure in the Ln2+xHf2−xO7−δ (Ln = Sm, Eu; x = 0.096) solid solutions and Ln2Hf2O7 (Ln = Gd, Tb) compounds prepared from mechanically activated oxide mixtures. Sm2.096Hf1.904O6.952 is shown to undergo pyrochlore-disordered pyrochlore-pyrochlore (P-P1-P) phase transformations in the temperature range 1200–1670°C. The former transformation leads to a rise in 840°C conductivity from 10−4 to 3 × 10−3 S/cm in the samples synthesized at 1600°C, and the latter leads to a drop in 840°C conductivity to 6 × 10−4 S/cm in the samples synthesized at 1670°C. The reduction in the conductivity of Sm2.096Hf1.904O6.952 is accompanied by the disappearance of the assumed superstructure. In the range 1300–1670°C, Eu2+xHf2−xO7−δ (x = 0.096) and Ln2Hf2O7 (Ln = Gd, Tb) have a disordered pyrochlore structure. The highest 840°C conductivity is offered by Eu2.096Hf1.904O6.952, Gd2Hf2O7, and Tb2Hf2O7 synthesized at 1670°C: 7.5 × 10−3, 5 × 10−3, and 2.5 × 10−2 S/cm, respectively.

Mechanochemical Synthesis of Wüstite, Fe1 – xO, in High-Energy Apparatuses

It is shown that high-energy milling of Fe2O3 + Fe mixtures leads to the formation of nanocrystalline, metastable wüstite Fe1 – xO. Its stoichiometry varies systematically with processing time.

Electrical conductivity of nanostructured fluorite-like Sc4Ti3O12

Single-crystal and polycrystalline samples of Sc4Ti3O12 have been shown to contain nanodomains (10–50 nm) with different degrees of ordering, coherent with the fluorite-like matrix. The oxygen-ion conductivity of this compound has been determined in the range 300–1000°C in air using impedance spectroscopy. The nanostructured single-crystal and polycrystalline samples are close in the activation energy for bulk conduction at both low and high temperatures: ≃1.26 and 1.29 eV in the range 300–775°C, ≃1.98 and 2.07 eV in the range 775–1000°C.

SYNTHESIS OF ZNO NANOTETRAPODS

ZnO tetrapods have been grown on silicon substrates by chemical vapor deposition, and the effect of synthesis conditions on their morphology and size has been studied. The cathodoluminescence spectra of the tetrapods show two emissions characteristic of ZnO, in the UV and green spectral regions. Their relative intensities depend on the vapor composition during synthesis and annealing conditions. A mechanism of tetrapod growth at significant supersaturations is discussed.

Molecular beam epitaxy of Pb1 − xEuxTe and Pb1 − xSnxTe layers and related periodic structures

Layers and periodic Bragg structures based on Pb1 − xEuxTe (0 < x < 1) and Pb1 − xSnxTe (0 < x < 0.1) ternary solid solutions have been grown on (111) BaF2 and (111) Si substrates by molecular beam epitaxy. X-ray diffraction measurements show that the layers in three-period EuTe/Pb0.94Eu0.06Te structures maintain the [111] crystallographic orientation normal to the substrate plane. In the interface plane, the [110] directions of the layers and substrate are parallel to each other. The full width at half maximum of the rocking curve of each layer of both the binary and ternary compounds is about 20′. The large optical contrast (40%) in such structures allowed us to reach 99.9% mid-IR reflectivity.

Electrical conductivity of Ln6–xZrxMoO12 + δ (Ln = La, Nd, Sm; x = 0.2, 0.6) ceramics during thermal cycling

In this paper, we analyze the relationship between the microstructure of new polycrystalline electron–proton conductors, Ln6–xZrxMoO12 + δ (Ln = La, Nd, Sm; x = 0.2, 0.6), and the reduction and hydration processes in these materials in humid atmospheres (air and argon). The La5.8Zr0.2MoO12.1 solid solution with a rhombohedral structure possesses not only the highest electrical conductivity among the materials studied here but also high stability in various dry and humid, oxidizing (air) and reducing atmospheres. La5.8Zr0.2MoO12.1 ceramic grains have a twin microstructure, and the conductivity of this material along the grain boundaries, consisting of ordered domains, differs little from its bulk conductivity. It seems likely that we observe a “domain wall” effect, typical of La0.95Sr0.05Ga0.9Mg0.1O3–δ (LSGM) oxygen ion conductors [1]. In studies of Ln6–xZrxMoO12 + δ (Ln = Nd, Sm; x = 0.2, 0.6) ceramics in humid atmospheres, we detected a grain-boundary contribution, which limited the total conductivity, like in perovskite BaZr0.8Y0.2O3–δ. We believe that such conditions lead to a reduction process in these materials and that Mo6+ is reduced before Nd3+ and Sm3+. The process first occurs on grain boundaries.

Oxygen Interstitial and Vacancy Conduction in Symmetric Ln 2 ± x Zr 2 ± x O 7 ± x/2 (Ln = Nd, Sm) Solid Solutions

We have compared (Ln2 − xZrx)Zr2O7 + x/2 (Ln = Nd, Sm) pyrochlore-like solid solutions with interstitial oxide ion conduction and Ln2(Zr2 − xLnx)O7 − δ (Ln = Nd, Sm) pyrochlore-like solid solutions with vacancy-mediated oxide ion conduction in the symmetric systems Nd2O3-ZrO2 (NdZrO) and Sm2O3-ZrO2 (SmZrO). We have studied their structure, microstructure, and transport properties and determined the excess oxygen content of the (Sm2 − xZrx)Zr2O7 + x/2 (x = 0.2) material using thermal analysis and mass spectrometry in a reducing atmosphere (H2/Ar-He). The Ln2 ± xZr2 ± xO7 ± x/2 (Ln = Nd, Sm) solid solutions have almost identical maximum oxygen vacancy and interstitial conductivities: (3–4) × 10−3 S/cm at 750°C. The lower oxygen vacancy conductivity of the Ln2(Zr2 − xLnx)O7 − δ (Ln = Nd, Sm; 0 < x ≤ 0.3) solid solutions is due to the sharp decrease in it as a result of defect association processes, whereas the interstitial oxide ion conductivity of the (Ln2 − xZrx)Zr2O7 + x/2 (Ln = Nd, Sm; 0.2 ≤ x < 0.48) pyrochlore-like solid solutions is essentially constant in a broad range of Ln2O3 concentrations.

Distinctive Features of the Formation of Geopolymer Aluminosilicate Materials

The chemical processes underlying the formation of hydroaluminosilicate geopolymer materials under various conditions from various mixtures of amorphous SiO2, metakaolin, sodium hydroxide, and water were studied. The results demonstrate that hardening at elevated temperatures is favorable for partial crystallization of the forming zeolite-like phases, whereas room-temperature hardening leads to the formation of X-ray amorphous phases. According to X-ray microanalysis data, the composition of the aluminosilicate matrix formed during hardening differs markedly from the intended composition. At the same time, the onset temperature for thermal dehydration (about 60–80°C) and hydrolysis kinetics varied little from sample to sample, which allowed us to tentatively determine the composition of the forming hydroaluminosilicate polymer.