Solid-state synthesis of undoped and Sr-doped K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>NbO<Subscript>3</Subscript>

The solid-state synthesis of undoped K_0.5Na_0.5NbO₃ (KNN) and KNN doped with 1, 2 and 6 mol% Sr, from potassium, sodium and strontium carbonates with niobium pentoxide, was studied using thermal analysis and in situ high-temperature X-ray diffraction (HT-XRD). The thermogravimetry and the differential thermal analyses with evolved-gas analyses showed that the carbonates, which were previously reacted with the moisture in the air to form hydrogen carbonates, partly decomposed when heated to 200 °C. In the temperature interval where the reaction was observed, i.e., between 200 and 750 °C, all the samples exhibited the main mass loss in two steps. The first step starts at around 400 °C and finishes at 540 °C, and the second step has an onset at 540 °C and finishes with the end of the reaction between 630 and 675 °C, depending on the particle size distribution of the Nb₂O₅ precursor. According to the HT-XRD analysis, the perovskite phase is formed at 450 °C for all the samples, regardless of the Sr content. The formation of a polyniobate phase with a tetragonal tungsten bronze structure was detected by HT-XRD in the KNN with the largest amount of Sr dopant, i.e., 6 mol% of Sr, at 600 °C.     

Effect of molten salt synthesis temperature on TiO<Subscript>2</Subscript> and Li cycling properties

In this project, we synthesized TiO₂ compounds through the molten salt method (MSM) using Ti(IV) oxysulfate, as the Ti source. Molten salts in the ratio of 0.375 M LiNO₃:0.180 M NaNO₃:0.445 M KNO₃ were added and heated at temperatures of 145, 280, 380, and 480 °C for 2 h in air, respectively. A part of the sample prepared at 145 °C was further reheated to 850 °C for 2 h in air. X-ray diffraction studies showed that the amorphous phase was obtained when the sample was prepared at 145 °C, and polycrystalline to crystalline anatase phase was formed when heated from 280 to 850 °C, which is complementary to the results of selected area electron diffraction studies. Electrochemical properties were studied using galvanostatic cycling, cyclic voltammetry, and electrochemical impedance spectroscopy at a current density of 33 mA g^?1 (0.1 C rate) and a scan rate of 0.058 mV s^?1, in the voltage range 1.0–2.8 V vs. Li. Electrochemical cycling profiles for the amorphous TiO₂ samples prepared at 145 °C showed single-phase reaction with a low reversible capacity of 65 mAh g^?1, whereas compounds prepared at 280 °C and above showed a two-phase reaction of Li-poor and Li-rich regions with a reversible capacity of 200 mAh g^?1. TiO₂ produced at 280 °C showed the lowest capacity fading and the lowest impedance value among the investigated samples.     

Thermal stability of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>

Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) of an α-Bi₂O₃ sample revealed staged phase transitions in the range 720–800°C (at 720, 780, and 800°C) and the elimination of oxygen to the composition Bi₂O_2.967 during heating to 895°C in air at 16 K/min. In dynamic vacuum (p = 1.33 Pa) at 780–800°C, Bi₂O₃ consecutively transforms to a phase with the cubic γ-Bi₂O₃ structure and tetragonal Bi₂O_2.3?2.4. In the latter, electron diffraction in a transmission electron microscope (ED/TEM) shows a superstructure with the superstructure vector q ₁₁₀ ≈ 1/9, which indicates an ordered arrangement of oxygen vacancies.     

Phase equilibria in the system Li<Subscript>2</Subscript>O-MgO-B<Subscript>2</Subscript>O<Subscript>3</Subscript>

The subsolidus region of the Li₂O-MgO-B₂O₃ system has been studied by X-ray powder diffraction and differential thermal analysis. Isothermal sections at 500–550 and 650–700°C have been designed. The following complex borates have been found to form: at 500–550°C, Li₂MgB₂O₅ and LiMgBO₃ are formed; at 650–700°C, a new phase Li₄MgB₂O₅ is formed along with LiMgBO₃; and at 5500–600°, Li₂MgB₂O₅ is formed.     

Study on the oxygen exchange capacities of Ce<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript> pyrochlore

Ce₂Sn₂O₇ pyrochlore was synthesized by a hydrothermal method. X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the composition and valence state of the sample. The oxygen exchange property of the Ce₂Sn₂O₇ phase was measured by an oxidation reaction in sealed air atmosphere and a followed reduction reaction in 5% H₂-95% N₂ atmosphere. Gas chromatography (GC) was used to analyze the oxygen change in the reaction. The results show that Ce₂Sn₂O₇ sample has excellent oxygen absorption capacity at 250°C as Ce³⁺ ions are oxidized to Ce⁴⁺ ions. The oxidized sample can be reduced by 5% H₂-95% N₂. The refreshed sample remains the capacity of oxygen absorption, while the oxygen exchange capacity degrades with the reduction times.     

Phase formation in mixed TiO<Subscript>2</Subscript>-ZrO<Subscript>2</Subscript> oxides prepared by sol-gel method

Pure titania, zirconia, and mixed oxides (3–37 mol.% of ZrO₂) are prepared using the sol-gel method and calcined at different temperatures. The calcined samples are characterized by Raman spectroscopy, X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption porosimetry. Measurements reveal a thermal stability of the titania anatase phase that slightly increases in the presence of 3–13 mol.% of zirconia. Practically, the titania anatase-rutile phase transformation is hindered during the temperature increase above 700°C. The mixed oxide with 37 mol.% of ZrO₂ treated at 550°C shows a new single amorphous phase with a surface area of the nanoparticles double with respect to the other crystalline samples and the formed srilankite structure (at 700°C). The anatase phase is not observed in the sample containing 37 mol.% of ZrO₂. The treatment at 700°C causes the formation of the srilankite (Ti_0.63Zr_0.37O_x) phase.     

High-Temperature X-Ray Diffraction Investigation of the Decomposition Process in Manganese-Gallium Spinel Mn<Subscript>1.5</Subscript>Ga<Subscript>1.5</Subscript>O<Subscript>4</Subscript>

The stability of spinel-type mixed Mn_1.5Ga_1.5O₄ oxide prepared in an inert medium (1000 °C, Ar) is studied by thermogravimetry and high-temperature X-ray diffraction in air in a wide temperature range 30–1000 °C. On heating, reversible decomposition processes of initial spinel are observed. From 30 °C to 600 °C oxygen atoms attach to the surface layer of initial Mn_1.5Ga_1.5O₄ spinel to form a new phase distinct from parent oxide by the oxygen stoichiometry (cation vacancies are formed). The product of decomposition is two oxides: Mn_1.5Ga_1.5O₄ and Mn_1.5–xGa_1.5–x[·]_xO₄. On the contrary, above 600 °C a loss of oxygen occurs, the concentration of cation vacancies decreases in Mn_1.5–xGa_1.5–x[·]_xO₄, and the reverse process of single phase oxide crystallization takes place. At 1000 °C the spinel phase forms again whose composition is similar to that of the initial parent phase Mn_1.5Ga_1.5O₄. On cooling the decomposition of this phase is again observed due to oxygen attachment.     

Changes in the surface structure of Pd/Ta2O5 by oxygen and CO studied using X‐ray Photoelectron Spectroscopy (XPS)

Behaviors of Pd structures with different thicknesses supported by Ta₂O₅/Ta in the reaction with oxygen and CO were studied by XPS and SEM. For the samples with a Pd thickness of 3 nm, a new low‐binding‐energy component appeared in the Pd 3d level upon O₂ exposure at ～200 °C and was reduced in intensity after a subsequent CO exposure at 150 and 200 °C. The change in the Ta 4f state could also be found upon oxygen and CO exposure, indicating that both Pd and the Ta‐oxide substrate participate in the chemical reactions. For the sample with a higher Pd thickness, a positive shift in the Pd 3d level due to the oxidation of Pd was observed after exposure to O₂ at a higher temperature (280 °C). A subsequent CO exposure at ～150 °C could not reduce Pd‐oxide layers, as confirmed by the unchanged Pd 3d spectra after CO treatment, i.e. Pd‐oxide was not reactive for CO oxidation. Copyright © 2010 John Wiley & Sons, Ltd.     

Co oxidation with oxygen in the presence of hydrogen on CoO/CeO<Subscript>2</Subscript> and CuO/CoO/CeO<Subscript>2</Subscript> catalysts

The catalytic activity of the CoO/CeO₂ and CuO/CoO/CeO₂ systems in selective CO oxidation in the presence of hydrogen at 20–450°C ([CuO] = 1.0–2.5%, [CoO] = 1.0–7.0%) is reported. The maximum CO conversion (X) decreases in the following order: CuO/CoO/CeO₂ (X = 98–99%, T = 140–170°C) > CoO/CeO₂ (X = 67–84%, T = 230–240°C) > CeO₂ (X = 34%, T = 350°C). TPD, TPR, and EPR experiments have demonstrated that the high activity of CuO/CoO/CeO₂ is due to the strong interaction of the supported copper and cobalt oxides with cerium dioxide, which yields Cu-Co-Ce-O clusters on the surface. The carbonyl group in the complexes Co^δ+-CO and Cu⁺-CO is oxidized by oxygen of the Cu-Co-Ce-O clusters at 140–160°C and by oxygen of the Co-Ce-O clusters at 240°C. The decrease in the activity of the catalysts at high temperatures is due to the fact that hydrogen reduces the clusters on which CO oxidation takes place, yielding Co⁰ and Cu⁰ particles, which are inactive in CO oxidation. The hydrogenation of CO into methane at high temperatures is due to the appearance of Co⁰ particles in the catalysts.     

Catalytic Activity of the Oxide Catalysts Based on Ni<Subscript>0.75</Subscript>Co<Subscript>2.25</Subscript>O<Subscript>4</Subscript> Modified with Cesium Cations in a Reaction of N<Subscript>2</Subscript>O Decomposition

The Ni_0.75Co_2.25O₄ catalysts were prepared by a coprecipitation method and modified with cesium cations by impregnation with a solution of cesium nitrate or cesium nitrate with citric acid and ethylene glycol additives (the Pechini method). The catalysts obtained were investigated by X-ray diffraction analysis, the BET method, X-ray photoelectron spectroscopy, temperature-programmed reduction, and the temperatureprogrammed desorption of oxygen. The activity of the samples in a reaction of nitrous oxide decomposition was determined at temperatures of 200–300°C, in particular, in the presence of oxygen and water in the reaction mixture. It was found that the use of the Pechini method for supporting Cs makes it possible to obtain a more active catalyst, as compared with that prepared by impregnation with cesium nitrate, at the same cesium content (~2%) of the samples.     

Electrochemical and magnetic characterization of LiFePO<Subscript>4</Subscript> and Li<Subscript>0.95</Subscript>Mg<Subscript>0.05</Subscript>FePO<Subscript>4</Subscript> cathode materials

A series of lithium iron phosphates was synthesized via the sol–gel route. Iron phosphides, which are electronic conductors, were formed when sintered at 850°C. Magnetic susceptibility measurements on the samples show antiferromagnetic behaviour with T _N=50±2 K for LiFePO₄ and Li_0.95Mg_0.05PO₄ sintered at temperatures below 850°C. The LiFePO₄ and Li_0.95Mg_0.05FePO4 cathodes show a stable electrochemical capacity in the range of 150–160 mA h/g on cycling. The cyclability deteriorates with increasing sample sintering temperature due to the increased crystal size and impurities.     

Two mechanisms of thermal expansion in perovskite SrCo<Subscript>0.6</Subscript>Fe<Subscript>0.2</Subscript>Nb<Subscript>0.2</Subscript>O<Subscript>3-z</Subscript>

Perovskite SrCo_0.6Fe_0.2Nb_0.2O_3-z attracts attention as a promising material with high oxygen conductivity. The sample was investigated by means of high-temperature X-ray powder diffraction and thermogravimetry. Phase transition was detected near 400 °C and accompanied with significant mass loss. The phase transition affects oxygen mobility, important for the synthesis of oxygen permeable membranes. The unit cell parameters are proved to change with temperature after two effects (1) reversible conventional thermal expansion and (2) irreversible contraction-expansion due to the changes in the oxygen content. In situ high-temperature X-ray diffraction experiments allowed us to separate the contributions and to measure them as a function of temperature.     

Electronic-Ionic Processes in Bi<Subscript>2</Subscript>Cu<Subscript>0.5</Subscript>Mg<Subscript>0.5</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript> with Pyrochlore Structure

Solid solution Bi₂Cu_0.5Mg_0.5Nb₂O_9–δ with the pyrochlore structure is synthesized by three different methods. Its structure and chemical composition are confirmed by X-ray diffraction analysis, electron microscopy, and energy-dispersive spectroscopy. The electronic-ionic processes are studied by the method of impedance spectroscopy in the frequency range from 0.3 Hz to 1.0 MHz and the temperature range from 0 to 340°С. The data are processed with the use of ZView program. Electrochemical models of samples are obtained in the form of equivalent circuits. The sign of the main charge carrier is determined by the thermo-emf method. Nonlinear effects are studied based on voltammetric characteristics. It is found that at room temperature, the charge in samples is transferred by electrons and cations (presumably, copper). In the temperature range of 260–300°С, the capacitance of samples and the specific conductivity of their volume demonstrate local minimums. Insofar as at these temperatures the oxygen conduction may occur, it is assumed that associates of anions and cations are formed. The decrease in the concentration of charge carries is confirmed by sample’s equivalent circuit into which the Gerischer impedance is introduced to enhance the accuracy. It is shown that at t = 260°С, the lifetime of charge carriers is the minimum.     

Controlled synthesis,characterization and thermal properties of Mg<Subscript>2</Subscript>B<Subscript>2</Subscript>O<Subscript>5</Subscript>

Optimum conditions for synthesizing monoclinic and triclinic Mg₂B₂O₅ compounds by high-temperature solid-state reactions were investigated. Mixtures composed of boric acid and magnesium oxide at MgO:B₂O₃ mole ratios of 1:0.25, 1:0.5 and 1:1.5 were heated for 1 hour at temperatures between 600–1050°C and the formed phases were identified by XRD analysis. Monoclinic Mg₂B₂O₅ was formed by heating at 850°C for 4 hours together with minimum amounts of triclinic Mg₂B₂O₅, while triclinic Mg₂B₂O₅ was formed as a single phase at 1050°C for the same reaction time. The products obtained at optimum conditions were subjected to a series of tests to determine their chemical compositions, particle size distributions, surface area values, IR spectra and TG/DTA patterns.      

High-temperature X-ray study of the formation and delamination of manganese-alumina spinel Mn<Subscript>1.5</Subscript>Al<Subscript>1.5</Subscript>O<Subscript>4</Subscript>

The behavior of the manganese-alumina system with Mn:Al = 1:1 on heating in air and vacuum was studied. The starting samples were mixtures of β-Mn₃O₄, α-Mn₂O₃, and γ-Al₂O₃. On heating to 950°C in air, the samples were partially oxidized into α-Mn₂O₃, and corundum α-Al₂O₃ formed along with mixed manganese-alumina cubic spinel, whose composition was close to Mn₂AlO₄. In vacuum at 1200°C, the starting sample with a ratio of Mn:Al = 1:1 transformed into the manganese-alumina spinel Mn_1.5Al_1.5O₄, which retained its cubic structure after slow cooling in vacuum. When cooled in air, this solid solution delaminated, and a nanocrystalline Mn_2.8Al_0.2O₄ phase formed, whose structure was β-Mn₃O₄ type tetragonal spinel.     

Nano particles of iron oxides in SiO<Subscript>2</Subscript> glass prepared by ion implantation

Quartz (SiO₂) glass was implanted with 5 × 10^{16 57}Fe ions/cm² at a substrate temperature of 500 °C, and annealed at temperatures between 700 and 950 °C. The implanted and annealed plates were characterized by conversion electron Mössbauer spectroscopy (CEMS), and measured by a Kerr effect magnetometer or a vibration sample magnetometer. Kerr effect measurement of as-implanted SiO₂ glass showed ferromagnetism at room temperature. CEM spectrum of the as-implanted glass consisted of magnetic relaxation peaks of finely dispersed metallic Fe species, and paramagnetic doublets of Fe³⁺ and Fe²⁺ species. The sample heated at 700 °C contained large grains of metallic Fe and a lot of oxidation products of Fe²⁺ species. After oxidation at temperatures higher than 800 °C, the samples showed also ferromagnetism, which was attributed mainly to ferromagnetic ε-Fe₂O₃ precipitated in SiO₂ matrix. Small amounts of α-Fe₂O₃ were produced at 950 °C. The results suggest that ion implantation and oxidation make a transparent ferromagnetic glass possible.     

Elimination of a zero-growth in thickness of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> protective film deposited by cycles of dip-coating method

Multilayered alumina film was deposited onto metallic substrate using cycles of dip-coating method. The film thickness was found not always growing linearly with the increase of the number of dipping cycles, and even a zero-growth in thickness was observed after 20 cycles of dip coatings. This phenomenon was found to be attributed to the dissolving behavior of alumina gel material in original sol. A heat treatment at a temperature higher than 230 °C was found to be able to effectively lower the dissolvability of Al₂O₃ gel material, but an extra high temperature, i.e., 600 °C led to the formation of cracks in the multilayered film due to the increase of interfacial tension force. It was examined by IR and XRD analyses that a heat treatment at 250 °C for 10 min before each coating process could yield an amorphous multilayered film with no crack formed after calcinations at 600 °C. A crack-free Al₂O₃ film with a thickness up to 2 μm after 22 cycles of dip coating process could be produced and it showed an excellent antioxidation performance for steel substrate.     

Mechanism of the Enhanced Electrochemical Properties of LiNi<Subscript>1/3</Subscript>Co<Subscript>1/3</Subscript>Mn<Subscript>1/3</Subscript>O<Subscript>2</Subscript> Cathode Materials by a Pre-Sintering Process

α-NaFeO₂ layered LiNi_1/3Co_1/3Mn_1/3O₂ cathode materials were synthesized by mechanical milling accompanied by the solid phase sintering. The sample exhibited a good crystallinity and layered structure while sintered at 900°C, which can be further improved by adding a pre-sintering process at 500°C before high temperature sintering. The sample with a pre-sintering process presents an average particle size about 0.6 μm, and a hexagonal crystalline structure. The optimally fabricated sample showed a first charge capacity of 210.2 mA h/g, discharge capacity of 171.2 mA h/g with a current rate of 0.2 C within the voltage range of 2.7~4.5 V. With increasing the current rate to 1 C, the charge–discharge capacity faded quickly during the cycling process, which can be partially recovered while operated at a low current rate. However, the capacity fading at a current rate of 2 C was largely irreversible. The evolution of the surface chemical states was evaluated using X-ray photoelectron spectroscopy on the charged and discharged samples to understand the high rate capacity fading.     

Formation of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–TiO<Subscript>2</Subscript> bilayer using atomic layer deposition and its application to dynamic random access memory

To enhance film conformality together with electrical property suitable for dynamic random access memory (DRAM) capacitor dielectric, the effects of oxidant and post heat treatment were investigated on aluminum and titanium oxide (Al₂O₃–TiO₂) bilayer (ATO) thin film formed by atomic layer deposition method. For the conformal deposition of Al₂O₃ thin film, the O₃ oxidant required a higher deposition temperature, more than 450 °C, while H₂O or combined oxygen sources (H₂O+O₃) needed a wide range of deposition temperatures ranging from 250 to 450 °C. Conformal deposition of the TiO₂ thin film was achieved at around 325 °C regardless of the oxidants. The charge storage capacitance, measured from the ATO bilayer (4 nm Al₂O₃ and 2 nm TiO₂) deposited at 450 °C for Al₂O₃ and 325 °C for TiO₂ with O₃ oxidant on the phosphine-doped poly silicon trench, showed about 15% higher value than that of 5 nm Al₂O₃ single layer thin film without any increase of leakage current. To maintain the improved electrical property of the ATO bilayer for DRAM application, such as enhanced charge capacitance without increase of leakage current, upper electrode materials and post heat treatments after electrode formation must be selected carefully. Dedicated to Professor Su-Il Pyun on the occasion of his 65th birthday.     

Synthesis of homogeneous La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> solid solutions by the Pechini method and their activity in methane oxidation

Homogeneous La_{1 − x} Ca _x MnO₃ solid solutions have been synthesized by the Pechini method (using polymer-solid compositions). Their microstructure, stability at high temperatures, and catalytic activity in methane oxidation are reported. A continuous series of solid solutions stable in air up to 1100°C forms in the system, and the particle surface is enriched with calcium. A distinctive microstructural feature of the particles is their microporosity. The catalytic activity of all calcium-containing samples (except for x = 0.7) below 700°C is lower than that of lanthanum manganite and decreases under the action of the reaction medium, which can be due to the decrease in the amount of weakly bound oxygen on the surface because of the enrichment of the surface with calcium and the formation of strongly bound surface carbonates. The higher activity and stability of the La_0.3Ca_0.7MnO₃ sample (calcined at 1100°C) above 500°C can be due to the formation of nanosized areas with an Mn₃O₄ structures on the perovskite particle surface in the reaction medium.