Preparation and characterization of V2O3 micro-crystals via a one-step hydrothermal process

Phase pure V₂O₃ micro-crystals with a hexagonal dipyramid morphology were fabricated for the first time via a facile one-step hydrothermal method. The crystals were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). A hexagonal dipyramid structure of V₂O₃ enclosed by well-grown {012} facets was obtained by hydrothermally reducing VO(acac)₂ precursor with N₂H₄·H₂O at 220 °C for 48 h. The results indicated that V₂O₃ can be well crystallized up to micron size with distinguished facets by only one step hydrothermal treatment. The formation mechanism and morphology evolution for V₂O₃ micro-crystals were discussed. Based on our experiments, the V₂O₃ nuclei formed and grew by a phase transformation through a dissolution–recrystallization process of VOOH, and the formation of the hexagonal dipyramids was ascribed to the specific adsorption of Hacac to the {012} facets restraining the growth in the directions normal to the {012} facets. The present work provides a facile method for preparing phase pure V₂O₃ micro-crystals with hexagonal dipyramid morphology, which can be used as a new powder material for ceramic fabrication.     

Characteristics of Y1Ba2Cu3O7-x high-T c superconductor prepared by partial melting process

The superconducting Y₁Ba₂Cu₃O_7-x (123) and the Y₂Ba₁Cu₁O₅(211) powders in this study were prepared by pyrophoric synthesis method with Y₂O₃ (99.9 %), BaCO₃ (99.9 %), and CuO (99.9 %) powders. Samples of 123 and 211 pellets were first prepared and then piled to have a 123/211/123 arrangement before a partial melting process was applied for phase change of the center piece (211) to 123 phase through a peritectic reaction. The process parameters were a melting temperature of 1,040–1,070 °C and the mass of the 123 piece ranging from 0.4 to 1.2 g. The superconductivity, such as critical temperature (T_c), and mass susceptibility(χ) of bulk 123 and 211 samples were measured by AC four point probe method and AC susceptometer, respectively. The experimental results can be summarized as follows: the best preparation condition in the range examined was a melting temperature of 1,060 °C and mass ratio [123(A)/211(B)] of 2:1 with melting time of 30 minutes to yield the T_c of 88.5 K.     

In situ synthesis,mechanical and cyclic oxidation properties of Ti3AlC2/Al2O3 composites

ABSTRACT

Ti₃AlC₂/Al₂O₃ composite materials were successfully fabricated from TiO₂/TiC/Ti/Al powders by the in situ reactive hot pressed technique. The microstructure, mechanical and oxidation properties of the composites were investigated in the paper. Vickers hardness increased with the Al₂O₃ content. The relative density of Ti₃AlC₂/Al₂O₃ composites exhibits a declining tendency with Al₂O₃ content especially exceeds 10 vol.?%. The Ti₃AlC₂/Al₂O₃ composites show excellent electrical conductivity. The flexural strength and fracture toughness of Ti₃AlC₂/10 vol. % Al₂O₃ are 461 ± 20?MPa and 6.2?±?0.2?MPa m^1/2, respectively. The cyclic oxidation behaviour of resistance of Ti₃AlC₂/10 vol. % Al₂O₃ composites at 800–1000°C generally obeys a parabolic law. The oxide scale of sample consists of a mass of α-Al₂O₃ and TiO₂, forming a dense and adhesive protect layer. The result indicates that the Al₂O₃ can greatly improve the oxidation resistance of Ti₃AlC₂.     

Hydrothermal synthesis of Li(Ni1/3Co1/3Mn1/3)O2 for lithium rechargeable batteries

Ultrafine powders of Li(Ni_1/3Co_1/3Mn_1/3)O₂ cathode materials for lithium-ion secondary batteries were prepared under mild hydrothermal conditions. The influence of the molar ratio of Li/(Ni + Co + Mn) was studied. The products were investigated by XRD, TEM and EDS. The final products were found to be well crystallized Li(Ni_1/3Co_1/3Mn_1/3)O₂ with an average particle size of about 10 nm.     

Photoluminescence properties of Eu3+-doped stalk-like Al2O3 via a hydrothermal route followed by heat treatment

ABSTRACT

Uniform Al₂O₃:Eu³⁺ samples were successfully fabricated via a hydrothermal method and subsequent thermal decomposition of Eu³⁺-doped precursors. The sample characterisations were carried out by means of X-ray diffraction (XRD), scanning electron microscope (SEM) and photoluminescence spectra. XRD results revealed Eu³⁺-doped samples were a pure γ-Al₂O₃ phase after being calcined at 1173?K. SEM results showed that these Eu³⁺-doped Al₂O₃ samples were stalk-like, with an average length of 1.5?μm. Upon excitation at 394?nm, the orange–red emission bands, having wavelengths longer than 580?nm, were to be from ⁵D₀→⁷F_J (J?=?1, 2) transitions. The asymmetry ratio of (⁵D₀→⁷F₂)/(⁵D₀→⁷F₁) intensity is about 0.54, 2.76, 3.29, 2.86, 3.36, 3.13 for Eu³⁺ concentrations of 0.1, 0.4, 0.7, 1.0, 1.5 and 2.0?mol-%, respectively. The optimal doping concentration of Eu³⁺ ions in Al₂O₃ is 1.5?mol-%. According to Dexter's theory, the critical distance between Eu³⁺ ions for energy transfer was determined to be 14?Å.     

Effect of polyvinylpyrrolidone on morphology and structure of In2O3 nanorods by hydrothermal synthesis

Indium oxide (In₂O₃) nanorods were hydrothermally synthesized from aqueous InCl₃ solution in urea with addition of polyvinylpyrrolidone (PVP) as a steric stabilizer. Indium hydroxide, In(OH)₃, was precipitated at 60 °C and was changed into a transient InOOH phase upon calcination at 250 °C in air. X-ray diffractometry revealed that the existence of PVP delays the phase transformation of InOOH. Cubic-structured In₂O₃ phase was then formed when temperature was raised to 350 °C, regardless of the PVP concentration. The In(OH)₃ phase without the PVP showed a rod-based, flower-like morphology of polycrystalline character. Minor addition of the PVP, i.e., 0.1–2 wt.%, resulted in a pronounced evolution in morphology from the three-dimensional, flower-like form to discrete, one-dimensional nanorods aligned in planar form. Both the flower-like and discrete nanorod morphologies were preserved after heat treatments at 250 and 350 °C. This reveals that the morphological change is attributable to preferential adsorption of the PVP molecules on the In(OH)₃ crystallite surface, so that the aggregate attachment responsible for the multipod growth is inhibited.     

Low-temperature synthesis of Bi2Fe4O9 nanoparticles via a hydrothermal method

Bi₂Fe₄O₉ (BFO) nanoparticles were successfully synthesized by a hydrothermal method at a temperature as low as 100 °C. The as-prepared powders, characterized by X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), transmission electron microscope (TEM) and physical property measurement system (PPMS), exhibited a pure BFO phase about 100 nm size with uniform sheet-like shape and exhibited an AF order at room temperature. It was found that high alkali concentration and alkali ion Na⁺ played a key role in the formation of BFO nanoparticles at a low temperature of 100 °C.     

Inductively coupled plasma etching of Pb(ZrxTi1-x)O3 thin films in Cl2/C2F6/Ar and HBr/Ar plasmas

Pb(Zr_xTi_1-x)O₃ thin films were etched in an inductively coupled plasma by using various etch gases such as Cl₂/Ar, C₂F₆/Ar, Cl₂/C₂F₆/Ar and HBr/Ar. The etch rates and etch profiles for each etch gas were investigated. Fast etch rates were obtained in chlorine-containing etch gases (e.g., Cl₂/Ar and Cl₂/C₂F₆/Ar), and clean and steep etch profiles were achieved in Cl₂/Cv₂F₆/Ar or HBr/Ar gases. The gas mixture of Cl₂ and C₂F₆ was proposed to give a fast etch rate and a steep sidewall angle of etched patterns. The optimum gas mixture of Cl₂C₂F₆/Ar was found by varying the gas ratio of Cl₂ to C₂F₆. On the other hand, HBr/Ar gas as an alternative for etching of the Pb(Zr_xTi_1-x)O₃ films was examined. Cl₂/C₂F₆/Ar and HBr/Ar etch gases were compared with respect to etch rate, etch profile and electrical properties.     

Annealing effect on relaxor behaviours of spark plasma sintered Pb(Sc1/2Nb1/2)O3 superfine ceramics

Abstract

Abstract

Relaxor ferroelectric Pb(Sc_1/2Nb_1/2)O₃ (PSN) superfine ceramics were fabricated by spark plasma sintering process and subjected to an annealing in oxygen ambience at a low temperature of 500°C. The microstructure of the PSN ceramics was examined by X‐ray diffraction and scanning and transmission electron microscopy, while the dielectric properties were measured in the range of 25–200°C. The dielectric measurements revealed a reverse change in the frequency dispersion ΔT and diffusive factor λ as annealing extended. Transmission electron microscopy observation and conductivity measurement revealed that the uncommon performance in relaxor properties could be attributed to the complex effects of oxygen vacancies and domain structures induced by the annealing.     

Rapid and low temperature synthesis of high purity Ti2SC powder by microwave hybrid heating

Ti₂SC powders were successfully synthesised at relatively low temperature by a microwave hybrid heating method. The mixtures of Ti, C and S or TiS₂ with different molar ratios were heated to investigate the formation of Ti₂SC phase. It was confirmed that Ti₂SC with high purity was achieved using TiS₂ as sulphur source. The results indicated that the synthesis temperature of Ti₂SC was ～500°C lower, and holding time was only one-twentieth compared with those reported by other methods. The high purity Ti₂SC powders from Ti/TiS₂/C with the molar ratio of 2.85:1.15:2 obtained at 1100°C for 3?min were uniform, and the average particle size was 1–2?μm.     

Preparation, characterization and dielectric properties of CaCu3Ti4O12 ceramics

CaCu₃Ti₄O₁₂ nano-sized powders were successfully prepared by sol-gel technique and calcination at 600-900 °C. The thermal decomposition process, phase structures and morphology of synthesized powders were characterized by IR, DSC-TG, XRD, TEM, respectively. It was found that the main weight-loss and decomposition of precursors occurred below 450 °C and the complex perovskite phase appeared when the calcination temperature was higher than 700 °C. Using above synthesized powders as starting materials, CCTO-based ceramics with excellent dielectric properties (?₂₅ = 5.9 × 10⁴, tan δ = 0.06 at 1.0 kHz) were prepared by sintering at 1125 °C. According to the results, a conduction mechanism was proposed to explain the origin of giant dielectric constant in CCTO system.     

Combustion synthesis of (Ti1−xNbx)2AlC solid solutions from elemental and Nb2O5/Al4C3-containing powder compacts

Preparation of the (Ti_1−xNb_x)₂AlC solid solution (formed from the M_n+1AX_n or MAX carbides, where n = 1, 2, or 3, M is an early transition metal, A is an A-group element, and X is C) with x = 0.2-0.8 was investigated by self-propagating high-temperature synthesis (SHS). Nearly single-phase (Ti,Nb)₂AlC was produced through direct combustion of constituent elements. Due to the decrease of reaction exothermicity, the combustion temperature and reaction front velocity decreased with increasing Nb content of (Ti_1−xNb_x)₂AlC formed from the elemental powder compacts. In addition, the samples composed of Ti, Al, Nb₂O₅, and Al₄C₃ were adopted for the in situ formation of Al₂O₃-added (Ti,Nb)₂AlC. The SHS process of the Nb₂O₅/Al₄C₃-containing sample involved aluminothermic reduction of Nb₂O₅, which not only enhanced the reaction exothermicity but also facilitated the evolution of (Ti,Nb)₂AlC. Based upon the XRD analysis, two intermediates, TiC and Nb₂Al, were detected in the (Ti,Nb)₂AlC/Al₂O₃ composite and their amounts were reduced by increasing the extent of thermite reduction involved in the SHS process. The laminated microstructure characteristic of the MAX carbide was observed for both monolithic and Al₂O₃-added (Ti,Nb)₂AlC solid solutions synthesized in this study.     

Hydrothermal synthesis of LiNixCo1−xO2 cathode materials

Ultrafine powders of LiCoO₂, nonstoichiometric LiNiO₂ and LiNi_0.9Co_0.1O₂ were prepared under mild hydrothermal conditions. The influence of the molar ratio of Li/Co, Li/Ni and Li/(Ni + Co) was studied. The final products were investigated by XRD, TEM and EDS. To synthesize a stoichiometric LiNiO₂ under mild hydrothermal conditions was found to be a big challenge. Transmission electron microscopies (TEM) revealed the formation of well-crystallized LiCoO₂ and LiNi_0.9Co_0.1O₂ with average size of 100 nm and 10 nm, respectively.     

Preparation of nanocrystalline Sr3Al2O6 powders via citric acid precursor

Sr₃Al₂O₆ was synthesized via citric acid precursor. The effects of the molar ratio of citric acid to total metal cations concentration (CA/M) on the formation of Sr₃Al₂O₆ were investigated. Increasing the CA/M promoted the formation of Sr₃Al₂O₆. Single-phase and well-crystallized Sr₃Al₂O₆ was obtained from the CA/M = 1, CA/M = 2 and CA/M = 4 precursor at temperature 1200 °C, 1100 °C and 900 °C, respectively. Differential thermal analysis and thermogravimetric (DTA/TG), X-ray diffractometry (XRD) and field emission scanning electron microscopy (FESEM) were used to characterize the precursors and the derived oxide powders. Sr₃Al₂O₆ nanoparticles with a diameter of about 50-70 nm were obtained.     

Interpretation of hydrothermal crystallization of fine BaTiO3 powders

In the preparation of fine BaTiO₃ powders under hydrothermal conditions, the reaction mechanism was interpreted through solid-state kinetic analysis of the Johnson-Mehl-Avrami plot. In this experiment reactants were dissolved and consumed to spherical particles of 50 nms from aggregation of several nanometer-sized particles. The particulate formation of BaTiO₃ underwent a 1st-order hydrolysis-condensation reaction with phase-boundary transition in the early stage of the reaction regardless of the initial concentration of the feedstock. However, as the concentration of nutrients was reduced, dissolution followed by precipitation became dominant, and a diffusioncontrolled reaction proceeded. When the concentration of nutrients was reduced to an extent that was not high enough to sustain supersaturation, the reaction was controlled by solidification for encapsulation of aggregated particles, inside of which the diffusion-controlled reaction slowly proceeded.     

Continuous hydrothermal synthesis of HT-LiCoO2 in supercritical water

A detailed investigation was made into the production of high temperature lithium cobalt oxide (HT-LiCoO₂) particles by continuous hydrothermal synthesis via the reaction of cobalt nitrate, lithium hydroxide, and hydrogen peroxide. The experiments were carried out in both subcritical and supercritical water, at temperatures ranging from 300 to 411 °C, with residence times less than 1 min in all instances. Although Co₃O₄ particles were synthesized in subcritical water at similar reaction conditions designed for comparison, well-ordered particles of HT-LiCoO₂ were obtained in supercritical water. In supercritical conditions, the variations in temperature and residence time did not have significant impacts on the average particle size, particle size distribution, or morphology of obtained HT-LiCoO₂. However, it was important to supply excessive lithium hydroxide and hydrogen peroxide in order to synthesize single-phased HT-LiCoO₂ particles without undesired by-products. The hydrothermal synthetic route for LiCoO₂, CoO, and Co₃O₄ in both subcritical and supercritical conditions was postulated.     

One step synthesis of lamellar R-3m LiCoO2 thin films by an electrochemical–hydrothermal method

In this study, we report the synthesis of lamellar R-3m LiCoO₂ thin films electrodes for lithium rechargeable batteries by a single step method based on an electrochemical–hydrothermal synthesis in a concentrated LiOH solution with a cobalt salt. This process combines the effect of temperature (between 150 °C and 200 °C), pressure and galvanostatic current. The obtained films were not annealed after the electrochemical–hydrothermal synthesis.For the first time, the theoretical study of the potential–pH diagram of cobalt was carried out at high temperature and high concentration. These calculations show that a pH value higher than 12 is necessary to avoid the direct precipitation of cobalt hydroxide Co(OH)₂ inside the solution. An improvement of the soluble species stability with an increase of the temperature and a decrease of the cobalt concentration is predicted. The influence of the deposition conditions (temperature and concentration) at a constant current density was experimentally studied. X-ray diffraction (XRD) shows the formation of well-crystallized LiCoO₂ thin films. Raman spectroscopy confirmed the achievement of the electrochemically active R-3m LiCoO₂ phase without any trace of the Fd3m phase at temperatures as low as 150 °C. Electrochemical measurements demonstrate good performances of the material synthesized between 150 °C and 200 °C with better capacity retention at higher temperature.     

Sintering of glass-added BaZn1/3Nb2/3O3 ceramics

The complex perovskite oxide Ba(Zn_1/3Nb_2/3)O₃ (BZN) has been studied for its attractive dielectric properties which place this material interesting for applications as multilayer ceramics capacitors or hyperfrequency resonators. This material is sinterable at low temperature with combined glass phase–lithium salt additions, and exhibits, at 1 MHz very low dielectric losses combined with relatively high dielectric constant and a good stability of this later versus temperature. The 2 wt.% of ZnO–SiO₂–B₂O₃ glass phase and 1 wt.% of LiF-added BZN sample sintered at 900 °C exhibits a relative density higher than 95% and attractive dielectric properties: a dielectric constant ?_r of 39, low dielectrics losses (tan(δ) < 10⁻³) and a temperature coefficient of permittivity τ_? of 45 ppm/°C⁻¹. The 2 wt.% ZnO–SiO₂–B₂O₃ glass phase and 1 wt.% of B₂O₃-added BZN sintered at 930 °C exhibits also attractive dielectric properties (?_r = 38, tan(δ) < 10⁻³) and it is more interesting in terms of temperature coefficient of the permittivity (τ_? = −5 ppm/°C). Their good dielectric properties and their compatibility with Ag electrodes, make these ceramics suitable for L.T.C.C applications.     

Mechanochemical synthesis and characterisation of BaTa2O6 ceramic powders

Mechanochemical synthesis was used to prepare BaTa₂O₆ powders from BaCO₃ and Ta₂O₅ precursors in a planetary ball mill. Effect of milling time and heat treatment temperature on the formation of BaTa₂O₆ and on the microstructure was investigated. Intensive milling of starting materials resulted in crystallization of BaTa₂O₆ even after 1 h of milling time and single phase BaTa₂O₆ was obtained after 10 h of milling under optimal conditions. The powder derived from 10 h of mechanical activation had crystallite size of 22 nm. But the increase in milling time did not decrease the crystallite size further. High energy milling activated the powders that although 1 h of milling led to formation of single phase BaTa₂O₆ at 1200 °C, this temperature decreased to 900 °C after 5 h of milling. No significant grain growth was observed when the milled powders were heat treated below 900 °C. However, annealing at 1100 and 1200 °C gave an average BaTa₂O₆ grain size of 180 and 650 nm, respectively. An unidentified phase started to form at 1100 °C increasing to high amounts at 1200 °C and they had different shapes and sizes than BaTa₂O₆ grains. These elongated large grains were thought to be due to liquid phase formation caused by iron contamination.     

Synthesis and properties of (BaxPb1−x)(Zn1/3Nb2/3)O3 relaxor ferroelectric ceramics using a reaction-sintering process

(Ba_xPb_1−x)(Zn_1/3Nb_2/3)O₃ (BPZN; x = 0.06–0.1) relaxor ferroelectric ceramics produced using a reaction-sintering process were investigated. Without any calcination involved, the mixture of raw materials was pressed and sintered directly. BPZN ceramics of 100% perovskite phase were obtained. Highly dense BPZN ceramics with a density higher than 98.5% of theoretical density could be obtained. Maximum dielectric constant K_max 13,500 (at 75 °C), 19,600 (at 50 °C) and 14,800 (at 28 °C) at 1 kHz could be obtained in 6BPZN, 8BPZN and 10BPZN, respectively. Dielectric maximum temperature (T_max) in BPZN ceramics via reaction-sintering process is lower than BPZN ceramics prepared via B-site precursor route.