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1.
The solid-state synthesis of undoped K0.5Na0.5NbO3 (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.  相似文献   

2.
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.  相似文献   

3.
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 Bi2O2.967 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 Bi2O2.3?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.  相似文献   

4.
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.  相似文献   

5.
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.  相似文献   

6.
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 (Ti0.63Zr0.37Ox) phase.  相似文献   

7.
The stability of spinel-type mixed Mn1.5Ga1.5O4 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 Mn1.5Ga1.5O4 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: Mn1.5Ga1.5O4 and Mn1.5–xGa1.5–x[·]xO4. On the contrary, above 600 °C a loss of oxygen occurs, the concentration of cation vacancies decreases in Mn1.5–xGa1.5–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 Mn1.5Ga1.5O4. On cooling the decomposition of this phase is again observed due to oxygen attachment.  相似文献   

8.
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.  相似文献   

9.
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 Coδ+-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.  相似文献   

10.
The Ni0.75Co2.25O4 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.  相似文献   

11.
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 Li0.95Mg0.05PO4 sintered at temperatures below 850°C. The LiFePO4 and Li0.95Mg0.05FePO4 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.  相似文献   

12.
Perovskite SrCo0.6Fe0.2Nb0.2O3-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.  相似文献   

13.
Solid solution Bi2Cu0.5Mg0.5Nb2O9–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.  相似文献   

14.
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.   相似文献   

15.
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 Mn1.5Al1.5O4, which retained its cubic structure after slow cooling in vacuum. When cooled in air, this solid solution delaminated, and a nanocrystalline Mn2.8Al0.2O4 phase formed, whose structure was β-Mn3O4 type tetragonal spinel.  相似文献   

16.
Quartz (SiO2) glass was implanted with 5 × 1016 57Fe 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.  相似文献   

17.
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.  相似文献   

18.
Homogeneous La1 − 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 La0.3Ca0.7MnO3 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.  相似文献   

19.
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.  相似文献   

20.
α-NaFeO2 layered LiNi1/3Co1/3Mn1/3O2 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.  相似文献   

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