Li2ZnTi3O8基微波介质陶瓷的研究进展

Li_2ZnTi_3O_8陶瓷是一种固有烧结温度低的新型微波介质陶瓷,具有较高的品质因数和近零的谐振频率温度系数。主要论述了近年来Li_2ZnTi_3O_8陶瓷制备方法、离子置换改性、氧化物掺杂改性以及低温共烧等方面的研究进展,指出了目前Li_2ZnTi_3O_8陶瓷研究中存在的问题及今后的发展趋势。     

Lithium insertion into αUO3 and U3O8

Lithium insertion compounds, Li_xUO₃ and Li_xU₃O₈, of two oxides of uranium have been prepared. Insertion of lithium has been carried out electrochemically and pure phases prepared by reaction of the metal oxides with LiI. Unit cell dimensions have been determined by Guinier powder X-ray analysis and are compared with those of the parent oxide.     

The Structural, Superconducting and Transport Properties of the Compounds Y3Ba5Cu8O18 and Y3Ba5Ca2Cu8O18

This work is related to the structural, superconducting, and transport properties of the compounds Y₃Ba₅Cu₈O₁₈ and Y₃Ba₅Ca₂Cu₈O₁₈ prepared by the sol-gel method. The influence of the doping of Ca atoms into the compound Y₃Ba₅Cu₈O₁₈ was studied by employing XRD, SEM, AFM, EDX, DTA, TGA, and the electrical resistivity (ρ), Hall coefficient (R _H), Hall mobility (μ _H), and magnetoresistance measurements. The XRD spectra showed that Y₃Ba₅Ca₂Cu₈O₁₈ almost has the same crystal structure as that of Y₃Ba₅Cu₈O₁₈, except with some impurity peaks. The resistivity measurements have pointed out that the compounds Y₃Ba₅Cu₈O₁₈ and Y₃Ba₅Ca₂Cu₈O₁₈ have their T _c-onset temperatures at approximately 92.7 and 86.6 K, respectively. The Hall coefficients R _H and Hall mobilities μ _H have been measured at the 10–300 K temperature interval in a magnetic field of 0.55 T. The signs of R _H and μ _H are found to be positive for both samples, which indicate that the conduction is p-type in our samples. As expected, the magnetoresistance results clearly demonstrate a considerable decrease of the offset temperatures with increasing magnetic field.     

Synthesis, structure and exchange bias in Cr2O3/CrO2/Cr2O5 particles

We report on the synthesis, structure and magnetic properties of a novel exchange bias system with Cr₂O₃/CrO₂/Cr₂O₅ interfaces. Chromium oxide particles with mixed chromium valences were prepared by sintering CrO₃ in air. X-ray diffraction patterns show that CrO₃ lost its oxygen gradually with increasing temperature and time through Cr₃O₈, Cr₂O₅, CrO₂, and finally Cr₂O₃ at temperatures above 760 K. X-ray photoelectron spectra indicate a low CrO₂ content and a binding energy of 579.3 eV for Cr 2p_3/2 photoelectrons in Cr₂O₅. Chromium dioxide was found to stably coexist with Cr₂O₃ and Cr₂O₅ in the particles. Magnetic measurements show hysteresis loop shifts in the sample, indicating an exchange bias induced by antiferromagnetic Cr₂O₃/Cr₂O₅ in ferromagnetic CrO₂. An exchange bias of 9 mT at 5 K and a coercivity of 26.3 mT were observed in the chromium oxide particles containing CrO₂.     

Preparation and characterization of Nb2O5-Al2O3 composite oxide formed by cathodic electroplating and anodizing

Al foil was coated with niobium oxide by cathodic electroplating and anodized in a neutral boric acid solution to achieve high capacitance in a thin film capacitor. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) revealed the niobium oxide layer on Al to be a hydroxide-rich amorphous phase. The film was crystalline and had stoichiometric stability after annealing at temperatures up to 600 °C followed by anodizing at 500 V, and the specific capacitance of the Nb₂O₅-Al₂O₃ composite oxide was approximately 27% higher than that of Al₂O₃ without a Nb₂O₅ layer. The capacitance was quite stable to the resonance frequency. Overall, the Nb₂O₅-Al₂O₃ composite oxide film is a suitable material for thin film capacitors.     

Surface and catalytic properties of MoO3/Al2O3 system doped with Co3O4

Thermal solid-solid interactions in cobalt treated MoO₃/Al₂O₃ system were investigated using X-ray powder diffraction. The solids were prepared by wet impregnation method using Al(OH)₃, ammonium molybdate and cobalt nitrate solutions, drying at 100 °C then calcination at 300, 500, 750 and 1000 °C. The amount of MoO₃, was fixed at 16.67 mol% and those of cobalt oxide were varied between 2.04 and 14.29 mol% Co₃O₄. Surface and catalytic properties of various solid samples precalcined at 300 and 500 °C were studied using nitrogen adsorption at −196 °C, conversion of isopropanol at 200-500 °C and decomposition of H₂O₂ at 30-50 °C.The results obtained revealed that pure mixed solids precalcined at 300 °C consisted of AlOOH and MoO₃ phases. Cobalt oxide-doped samples calcined at the same temperature consisted also of AlOOH, MoO₃ and CoMoO₄ compounds. The rise in calcination temperature to 500 °C resulted in complete conversion of AlOOH into very poorly crystalline γ-Al₂O₃. The further increase in precalcination temperature to 750 °C led to the formation of Al₂(MoO₄)₃, κ-Al₂O₃ besides CoMoO₄ and un-reacted portion of Co₃O₄ in the samples rich in cobalt oxide. Pure MoO₃/Al₂O₃ preheated at 1000 °C composed of MoO₃-αAl₂O₃ solid solution (acquired grey colour). The doped samples consisted of the same solid solution together with CoMoO₄ and CoAl₂O₄ compounds.The increase in calcination temperature of pure and variously doped solids from 300 to 500 °C increased their specific surface areas and total pore volume which suffered a drastic decrease upon heating at 750 °C. Doping the investigated system with small amounts of cobalt oxide (2.04 and 4 mol%) followed by heating at 300 and 500 °C increased its catalytic activity in H₂O₂ decomposition. This increase, measured at 300 °C, attained 25.4- and 12.9-fold for the solids precalcined at 300 and 500 °C, respectively. The increase in the amount of dopant added above this limit decreased the catalytic activity which remained bigger than those of un-treated catalysts. On the other hand, the doping process decreased the catalytic activity of treated solids in isopropanol conversion especially the catalysts precalcined at 300 °C. This treatment modified the selectivities of treated solids towards dehydration and dehydrogenation of reacted alcohol.The activation energies of H₂O₂ decomposition were determined for pure and variously doped solids. The results obtained were discussed in light of induced changes in chemical composition and surface properties of the investigated system due to doping with cobalt oxide.     

Caracterisation physico-chimique du ferrite de calcium et de lanthane Ca2LaFe3O8

In the Ca₂Fe₂O₅LaFeO₃ system a new ferrite of formula Ca₂LaFe₃O₈ has been isolated. As its precise cristallographic structure is not known, the structural model previously proposed has been tested by Mössbauer spectroscopy and magnetic measurements. The Fe³⁺ ions occupy one tetrahedral and two octahedral sites. The magnetic behaviour of Ca₂LaFe₃O₈ is close to those of Ca₂Fe₂O₅ and LaFeO₃. The Néel temperature is 735 K. The various exchange integrals have been calculated using a method previously applied to Ca₂Fe₂O₅.     

Effect of Co0.58Ni0.42 oxide nanoneedles coating on the electrochemical properties of LiV3O8 cathode

LiV₃O₈ has been coated by various amounts of Co_0.58Ni_0.42 oxide nanoneedles using a chemical co-precipitation method. The influences of the coating on the structure, morphology, and electrochemical properties of LiV₃O₈ have been characterized by X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), and electrochemical measurements. Co_0.58Ni_0.42 oxide coated LiV₃O₈ materials exhibit distinct surface morphology. The coating has been found to improve the electrochemical performance of LiV₃O₈ significantly. Especially, 5.0 wt.% Co_0.58Ni_0.42 oxide nanoneedles coated LiV₃O₈ shows much better electrochemical performance than uncoated LiV₃O₈. It has been proved that Co_0.58Ni_0.42 oxide coating suppresses phase transitions and decreases the charge-transfer impedance of LiV₃O₈ effectively.     

Low-temperature synthesis of LiV3O8 nanosheets as an anode material with high power density for aqueous lithium-ion batteries

Nanosheets of lithium vanadium oxide (LiV₃O₈) were successfully synthesized by a simple low temperature citrate sol–gel combustion route. Compact nanosheets of the active material were observed by scanning and transmission electron microscopies. X-ray diffraction measurements indicated that as-prepared nanosheets presented pure phase of monoclinic LiV₃O₈ with p2₁/m symmetry. Cyclic voltammetry (CV) was employed to investigate the electrochemical behavior of the nanosheets with special emphasis on the application potential as anodic material for aqueous rechargeable lithium batteries. CV studies demonstrated that the LiV₃O₈ nanosheets represent well-defined reversible peaks. The nanosheets showed a discharge capacity of 63 mAh/g in 1.0 M LiNO₃ solution at a 2C/5 rate.     

The formation of ultrafine Al2O3, ZrO2 and Fe2O3 by the spray-ICP technique

Ultrafine aluminium oxide, zirconium oxide and iron oxide have been prepared by spraying their corresponding nitrate solutions into an inductively coupled plasma (ICP) of ultra-high temperature (the spray-ICP technique). Their particles are roundish, 0.01–0.02 μm in diameter, and homogeneous both in size and in shape. Their major phases are γ-Al₂O₃, metastable tetragonal-ZrO₂ and γ-Fe₂O₃, respectively. These oxides appear to be formed through the following two reaction processes, (1) the complete decomposition of the droplets of the solution to their component atoms in the ICP flame, (2) the homogeneous nucleation and the successive crystal growth occurring outside the ICP flame. In the process (2), quenching is so rapid that the crystals grown after the nucleation do not transform to high temperature phases, which seems to cause the formation of the oxide of low temperature phase.     

Preparation and electrochemical properties of Cr doped LiV3O8 cathode for lithium ion batteries

Chromium was incorporated into lithium trivanadate by an aqueous reaction followed by heating at 100 °C. This Cr doped LiV₃O₈ as a cathode for lithium ion batteries exhibits 269.9 mAh g^− 1 at first discharge cycle and remains 254.8 mAh g^− 1 at cycle 100, with a charge-discharge current density of 150 mA g^− 1 in the voltage range of 1.8-4.0 V. The Cr-LiV₃O₈ cathode show excellent discharge capacity, with the retention of 94.4% after 100 cycles. These result values are higher than previous reports indicating that Cr-LiV₃O₈ prepared by our low temperature synthesis method is a promising cathode material for rechargeable lithium ion batteries. The enhanced discharge capacity and cycle stability of Cr-LiV₃O₈ cathode indicate that chromium atoms promote lithium transfer or intercalation/deintercalation during the electrochemical cycles and improve the electrochemical performances of LiV₃O₈ cathode.     

Complete, reversible H/Li ion exchange reaction between rhombohedral LiMO3 and perovskite-type HMO3 (M=Nb, Ta)

We demonstrate for the first time the complete, reversible H⁺/Li⁺ ion exchange reaction between HMO₃ and LiMO₃ (M=Nb, Ta) using molten LiNO₃ at 320°C for 5 days. HMO₃ were prepared from LiMO₃ by ion exchange reactions using dilute HNO₃. The results reveal that both LiMO₃ prepared by normal solid state reaction between Li₂CO₃ and M₂O₅, and by ion exchanged from HMO₃ using molten LiNO₃ are isostructural. LiMO₃ prepared using HMO₃ yields nearly uniformly sized crystallites in contrast to those prepared by conventional solid state synthesis. The present method is simple and inexpensive compared to other methods of preparation of high purity LiMO₃ powders.     

Hydration of ZrW2O8 nanopowders under ambient conditions

We report hydration of ZrW₂O₈ under ambient conditions, and its effect on the negative coefficient of thermal expansion (CTE) of ZrW₂O₈. On storing under ambient conditions for six months, about 66% of the outer annular volume was hydrated to ZrW₂O₈·0.35H₂O, while after one year of storage the sample was hydrated to ZrW₂O₈·0.72H₂O. The CTE was determined by in-situ high temperature X-ray diffraction (XRD) measurements in the temperature range from 25 to 200 °C. XPS and TGA were used to characterize the nature of bonding of the water in the ZrW₂O₈ structure. The negative CTE behavior of partially hydrated ZrW₂O₈·0.35H₂O remained intact, while on further hydration to ZrW₂O₈·0.72H₂O, negative CTE was not observed. The bonding of the water molecules to the ZrW₂O₈ structure were stronger than those of adsorbed water molecules but weaker than those of the structural hydroxyl ions.     

Formation process of Al2O3 thin film by reactive sputtering

Al and Al₂O₃ films were deposited by RF magnetron sputtering using a mixed gas of Ar and O₂. The surface of the Al target was changed from the metallic mode to the oxide mode at a critical O₂ flow ratio of 8%. The atomic ratio of sputtered Al atoms to supplied oxygen atoms was found to be approximately 2:3 at the critical O₂ flow ratio. The oxide layer thickness formed on the Al target was estimated to be 5-7 nm at an O₂ flow ratio of 100% by ellipsometry.     

Development of a modified CVD coating process for the enhancement of the high temperature oxidation resistance of Cr2O3 and Al2O3 forming alloys

In this work, a new deposition method for coating Cr₂O₃ and Al₂O₃ forming alloys has been set up. This is a modified CVD deposition method based on the pyrolisis of an aerosol produced by an ultrasonic wave focussed on the surface of a solution containing a reactive element soluble salt that will later transform to an oxide. The deposition takes place in a reaction chamber maintained at 473 K. To test the efficiency of the deposition method, commercial AISI-304 stainless steel samples were coated directly on the mill finish surface and on a pre-formed oxide layer with lanthanum nitrate. Specimens were then oxidised at 1173 K in synthetic air atmosphere during 25 h in a thermobalance. Oxidation kinetics of coated samples was compared with that of untreated ones. The results show that coated samples improve their high temperature oxidation resistance by decreasing their parabolic rate constant by one order of magnitude. The deposition method developed proves to be an easy, cheap and efficient way of increasing the service temperature of conventional stainless steel.     

Influence of TiO2 incorporation in HfO2 and Al2O3 based capacitor dielectrics

Atomic layer deposition was applied to fabricate metal oxide films on planar substrates and also in deep trenches with appreciable step coverage. Atomic layer deposition of Ru electrodes was realized on planar substrates. Electrical and structural behaviour of HfO₂-TiO₂ and Al₂O₃-TiO₂ nanolaminates and mixtures as well as Al₂O₃ films were evaluated. The lowest leakage current densities with the lowest equivalent oxide thickness were achieved in mixed Al₂O₃-TiO₂ films annealed at 700 °C, compared to all other films in as-deposited state as well as annealed at 900 °C. The highest permittivities in this study were measured on HfO₂-TiO₂ nanolaminates.     

Alkaline earth titanoniobates with a layer structure : A0.5H(TiNbO5)2.nH2O

The ion exchange properties of the layer oxide HTiNbO₅ in the presence of alkaline earth acetate solution A(CH₃COO)₂ has been investigated. The maximum exchange rate increases with the size of the A ion, leading to the hydrates : Ba(TiNbO₅)₂.4H₂O, Sr_0.75H_0.50(TiNbO₅)₂.4H₂O and Ca_0.50H(TiNbO₅)₂.4H₂O. The minimum exchange rate corresponds to the definite compounds A_0.5H(TiNbO₅)₂.4H₂O, labelled 4W-hydrates. The study of the thermal stability of the 4W-hydrates leads to the hydrates A_0.5H(TiNbO₅)₂.2H₂O and A_0.5H(TiNbO₅)₂.H₂O, and finally to the dehydrated oxides A_0.5H(TiNbO₅)₂. The reversibility of the ion exchange reaction and of the hydration phenomenon has been studied. The X-ray diffraction study of these compounds shows that the structure of the (TiNbO₅) layers is conserved whatever the nature of the A ion and of the hydrate may be. The structural evolution versus water content and size of the A ions is discussed.     

A phase relation between congruently melting LiNbO3 and Fe2O3

A pseudo-binary phase relation study has been done on the congruently melting LiNbO₃ and Fe₂O₃. DTA and X-ray analyses show that a solid solution exists between 0 and 11 mol% Fe₂O₃. The hexagonal cell constants a_H and V_H decrease with increasing Fe³⁺ content, whereas the c_H axis becomes larger up to 6 mol% Fe₂O₃ and then contracts. This anomalous behaviour in the c_H is explained by a preferential substitution of Nb⁵⁺ for Fe³⁺.     

Synthesis of Sb2O3 nanorods under hydrothermal conditions

Sb₂O₃ nanorods were successfully prepared via a mild hydrothermal route based on the reactions between SbCl₃ and NH₃·H₂O in aqueous solution at 120-180 °C for 12 h. The as-prepared Sb₂O₃ nanorods were characterized by X-ray diffraction (XRD), transmission electronic microscopy (TEM), and X-ray photoelctron spectroscopy (XPS). Results showed that NH₃·H₂O played a significant role in the formation of Sb₂O₃ nanorods. The presence of NH₃·H₂O could greatly favor the reaction progress toward the right-hand side and led to the orientation growth of Sb₂O₃ nanorods. A possible mechanism for the formation of Sb₂O₃ nanocrystallites was discussed.     

Improvement of Al2O3/Ge interfacial properties by O2-annealing

The electrical properties of an Al₂O₃/Ge gate stack structure were improved by O₂-annealing. The interface state density can be decreased by O₂-annealing without the formation of a GeO₂ interfacial layer. X-ray photoelectron spectroscopy measurements revealed that Ge diffusion into the Al₂O₃ layer occurs and Ge is uniformly distributed in the oxide layer after O₂-annealing. Crystallization of the Al₂O₃ film was observed after O₂-annealing at 550 °C and was identified as an Al-Ge-O compound using cross sectional transmission electron microscopy and transmission electron diffraction measurement.