LaCrO<Subscript>3</Subscript> powder from lanthanum trisoxalatochromate(III) (LTCR) precursor

Lanthanum chromite LaCrO₃, an important catalyst and interconnect material used in solid oxide fuel cell was prepared from lanthanumtrisoxalatochromate(III) hydrate [LaCr(C₂O₄)₃]·9H₂O (LTCR) employing microwave heating technique. The compound LTCR heated in microwave heating system gave pure LaCrO₃ at 500°C within one hour. However LTCR heated in silicon carbide furnace yielded LaCrO₃ at 900°C. BET surface area of LaCrO₃ prepared by microwave and conventional heating techniques were found to be 2.8 and 1.2 m² g⁻¹, respectively. Thermogravimetry, differential thermal analysis and X-ray diffraction techniques were used to optimize the conditions for the microwave processing of the precursor.     

Microwave assisted low temperature synthesis of sodium zirconium phosphate (NaZr<Subscript>2</Subscript>P<Subscript>3</Subscript>O<Subscript>12</Subscript>)

Sodium zirconium phosphate [NaZr₂P₃O₁₂], a potential ceramic matrix for fixation of high level nuclear waste, was synthesized by heating the mixture of sodium carbonate [Na₂CO₃], zirconyl nitrate hydrate [ZrO(NO₃)₂·5H₂O] and ammonium dihydrogen phosphate [NH₄H₂PO₄] in air, in a resistance heated furnace and a microwave heating system respectively in the temperature range 450 to 650°C. The mixture heated for 1 h in a resistance furnace at 450°C yielded a poorly crystalline NaZr₂P₃O₁₂ [NZP]. Increasing the temperature to 650°C produced a highly crystalline product. The same mixture heated in a microwave oven at 450°C for 1 h however, yielded the most crystalline NZP.In an alternate method, the mixture of sodium dihydrogen phosphate (NaH₂PO₄), zirconium dioxide (ZrO₂) and diammonium hydrogen phosphate [(NH₄)₂HPO₄] heated in resistance furnace at 650°C for the same period did not react in air. It also did not yield the pure product at 450°C when heated in microwave assembly for 1 h.The authors thank the Board of Research in Nuclear Sciences (BRNS) of the Department of Atomic Energy (DAE) for the financial support for this work under the project No. 2000/37/19/BRNS/1959 dtd09-02-02.     

Nanosized SrTiO<Subscript>3</Subscript> powder from oxalate precursor microwave aided synthesis and thermal characterization

Nanosized strontium titanate (SrTiO₃) was synthesized from strontium titanyl oxalate hydrate, SrTiO(C₂0₄)₂4H₂0 (STO) precursor employing microwave heating technique. STO precursor was characterized by Thermogravimetry (TG) and differential thermal analysis (DTA) techniques prior to the heat treatment in conventional and microwave heating system. STO precursor heated in microwave heating system in air at 773 K for 30 min yielded pure cubic SrTiO₃. The product obtained by heating of STO precursor in the same system at 973 K for same duration was, however, much more crystalline. Experiments repeated in conventional furnace showed that SrTiO₃ was formed above 973 K. SrTiO₃ powder obtained was characterized by X-ray diffraction (XRD) and Transmission electron microscopy (TEM) techniques. TEM study shows that the particles of SrTiO₃ are nearly spherical in shape and the particle size of SrTiO₃ powder varies between 28 and 68 nm.     

Dielectric and tunable properties of highly (110)-oriented (Ba<Subscript>0.65</Subscript>Sr<Subscript>0.35</Subscript>)TiO<Subscript>3</Subscript> thin films deposited on Pt/LaNiO<Subscript>3</Subscript>/SiO<Subscript>2</Subscript>/Si substrates

Highly (110)-oriented Ba_0.65Sr_0.35TiO₃ films were deposited on Pt/LaNiO₃/SiO₂/Si substrates by a sol–gel method. It was found that the (110)-preferred Pt film was very effective for growing (110)-oriented ferroelectric films with perovskite structure. The as-grown Ba_0.65Sr_0.35TiO₃ films showed good dielectric properties with dielectric constant and loss tangent tan δ = 0.026. Excellent dielectric tunability was also achieved in the (110)-oriented films. With applying an electric field of 230 kV/cm at 100 kHz, the dielectric tunability and the figure of merit can reach up to 63.4% and 16, respectively. These results indicate that the highly (110)-oriented Ba_0.65Sr_0.35TiO₃ film is a promising candidate for the applications in microwave tunable devices.     

Thermodynamic,thermogravimetric and permittivity studies of hausmannite (Mn<Subscript>3</Subscript>O<Subscript>4</Subscript>) in air

Some manganese oxides are considered hyperactive under microwave irradiation because of their extremely high heating rates in air. In order to further understand this hyperactivity, thermodynamic calculations, thermogravimetric analysis and both real and imaginary permittivity determinations were performed for hausmannite (Mn₃O₄) as a function of temperature in an air atmosphere. The thermodynamic results demonstrated reasonable agreement with the thermogravimetric analysis data. A comparison of the derivative thermogravimetric analysis data with the derivative of both the real and the imaginary permittivities confirmed that the extremely high values of the permittivities were due to the conversion of the hausmannite to bixbyite (Mn₂O₃). The microwave hyperactivity of the manganese oxides in air is explained in terms of the high permittivities of bixbyite.     

Dilatometry and high temperature x-ray diffractometry study of LaCrO<Subscript>3</Subscript> prepared using microwave heating

The phase transitions in the LaCrO₃ were studied using bulk dilatometry and high temperature X-ray diffractometry from room temperature to 1050 and 1200°C, respectively. LaCrO₃ was prepared at 500°C from oxalate precursor employing microwave heating technique. Bulk shrinkage measurements on LaCrO₃ pellets were carried out using dilatometer designed and fabricated in our own laboratory. Dilatometric curves of LaCrO₃ showed two peaks in ΔL/L vs. temperature curves in the range 200–400 and 800–1000°C, respectively. These phase transitions have been confirmed using high temperature X-ray diffractometry. The role of simple technique like bulk dilatometry in detecting and monitoring the polymorphic transformations in solids is discussed for lanthanum chromates.     

Generation of fullerenyl radicals and chemiluminescence in the (C<Subscript>60</Subscript>—R<Subscript>3</Subscript>Al)—O<Subscript>2</Subscript> system

Fullerenyl radicals (FR) RC₆₀ ^· and chemiluminescence (CL) are generated in the presence of O₂ in C₆₀—R₃Al (R = Et, Buⁱ) solutions in toluene (T = 298 K). The FR are formed due to the addition of the R^· radical, which is an intermediate of R₃Al autooxidation, to C₆₀. Mass spectroscopy and HPLC were used to identify Et_nC₆₀H_m (n, m = 1–6), Et_pC₆₀ (p = 2–6), and dimer EtC₆₀C₆₀Et as stable products of FR transformations. As found by ESR, the EtC₆₀ ^· radical (g = 2.0037) is also generated by photolysis of solutions obtained after interaction in the (C₆₀— R₃Al)—O₂ system. In the presence of dioxygen, the FR is not oxidized but yields complexes with O₂, which appear as broadening of the ESR signals. Chemiluminescence arising in the (C₆₀—R₃Al)—O₂ system is much brighter (I _max = 1.86·10⁸ photon s⁻¹ mL⁻¹) than the known background CL (I _max = 6.0·10⁶ photon s⁻¹ mL⁻¹) for the autooxidation of R₃Al and is localized in a longer-wavelength spectral region (λ_max = 617 and 664 nm). This CL is generated as a result of energy transfer from the primary emitter ³CH₃CHO* to the products of FR transformation: R_nC₆₀H_m, R_pC₆₀, and EtC₆₀C₆₀Et. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 205–213, February, 2007.     

Lithium intercalation into CrO<Subscript>3</Subscript>-GIC and its derivatives

Graphite intercalation compound with chromium trioxide (CrO₃-GIC) was used as a precursor for new graphite composites. By heating CrO₃-GIC at 800 °C in air a composite material, expanded graphite/Cr₂O₃, was obtained, whereas further heating of this composite at 1000 °C in the stream of hydrogen/argon mixture led to the reduction of Cr₂O₃ to chromium carbide (Cr₃C₂). A new material thus obtained was expanded graphite/Cr₃C₂ composite. The chromium species having catalytic properties gave rise to the improvement of the crystal structure of expanded graphite. All materials were tested as anode materials for lithium-ion cell. It was found that graphitic material treated in H₂/Ag at 1000 °C has greater reversible capacity in comparison to the original graphite and the shape of the charge/discharge curve shows very well-defined stage structure which supports the crystallographic data.Presented at the 3rd International Meeting Advanced Batteries and Accumulators, June 16th–June 20th, Brno, Czech Republic     

Schiff base complex sol–gel method for LaCoO3 perovskite preparation with high-adsorbed oxygen

A novel Schiff base complex sol–gel method has been used to prepare LaCoO₃ producing high ratios of adsorbed (or surface) oxygen (α) to lattice oxygen (β). The as-prepared gels, characterized by Fourier transform infrared spectroscopy (FTIR), showed that both lanthanum and cobalt ions were complexed before calcinations. IR spectra revealed that CO₃²⁻ and NO₃⁻ presented on the sample surfaces during heat treatment. High-resolution transmission electron microscopic (HRTEM) images of all samples showed resolved lattice fringes with the inter-planar spacing 0.37–0.39 nm of the (0 1 2) plane in hexagonal perovskite. BET surface areas of LaCoO₃ nano-crystals were 11.7–18.6 m²/g. Ratios of adsorbed (or surface) oxygen (α) to lattice oxygen (β) quantified by X-ray photoemission spectroscopy showed that LaCoO₃ prepared by the Schiff base complex method produced higher ratios when bases had higher nitrogen content in molecules. Carbonate and nitrate which were resulting from the oxidation of functional groups in the Schiff base complex, can produce gaseous compounds and leave vacant sites for oxygen in the gas phase to adsorb.     

Microwave-activated carbon dioxide reforming of propane over Ni/TiO<Subscript>2</Subscript> catalysts

The use of microwave activation in Ni/TiO₂-catalyzed carbon dioxide reforming of propane increases the catalytic activity and significantly reduces the coke formation in comparison with conventional thermal heating. During microwave activated reaction, C₂—C₃ olefins were formed, apart from CO and H₂, and the selectivity to olefins reached 6%. It was suggested that exposure to microwave radiation may induce local high-temperature heating of catalytically active phases and catalyst sites, which is not inherent in conventional heating. According to X-ray absorption spectroscopy (XAS = XANES + EXAFS), unlike conventional thermal heating in a hydrogen flow, on exposure to microwave radiation, the Ni²⁺ cations are partly reduced to Ni⁰.     

Thermal behavior of Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> mesoporous gels

Summary Titania-based photocatalytic materials were prepared by sol-gel method using Fe³⁺ and polyethyleneglycol (PEG₆₀₀) as additives. Thermogravimetry (TG), differential thermal analysis (DTA) and evolved gas analysis (EGA) with MS detection were used to elucidate processes that take place during heating of Fe³⁺ containing titania gels. The microstructure development of the Fe₂O₃/TiO₂ gel samples with and without PEG₆₀₀ admixtures was characterized by emanation thermal analysis (ETA) under in situ heating in air. A mathematical model was used for the evaluation of ETA results. Surface area and porosity measurements of the samples dried at 120&deg;C and the samples preheated for 1 h to 300 and 500&deg;C were compared. From the XRD measurements it was confirmed that the crystallization of anatase took place after thermal heating up to 600&deg;C.     

Kinetics study on the dissolution of UO<Subscript>2</Subscript> particles by microwave and conventional heating in 4 mol/L nitric acid

The dissolution of UO₂ particles in 4 mol·L⁻¹ nitric acid medium at temperatures of 90–110°C by microwave heating and conventional heating has been investigated, respectively. It is found that the dissolution ratios of UO₂ particles by microwave heating were 10%–40% higher than that by conventional heating. Kinetics research shows that the dissolution of UO₂ particles in 4 mol·L⁻¹ nitric acid is controlled by the diffusion control model for microwave heating and by the surface reaction control model for conventional heating. The diffusion control model for the dissolution of UO₂ particles by microwave heating could be explained by the diffuseness on the surface of UO₂ particles.     

Preparation and Characterisation of Nano-Structured WO<Subscript>3</Subscript>-TiO<Subscript>2</Subscript> Layers for Photoelectrochromic Devices

Nano-structured WO₃-TiO₂ layers were prepared by the sol-gel route. To obtain transparent, porous and crack free layers up to 0.8 μ m with a single dipping cycle a templating strategy was used. As a template three-dimensionally network based on organically modified silane was introduced to the WO₃ and TiO₂ sols. The WO₃ layers were dip-coated onto the conductive glass substrate (TCO) and the TiO₂ layers on the top of the WO₃ layer. The morphology and the structure of the layers were determined by Scanning Electron Microscopy (SEM), High Resolution Transmission Electron Microscopy (HR-TEM), Energy Dispersive X-Ray Spectroscopy (EDXS), Auger and Infrared spectroscopy. SEM image of the WO₃-TiO₂ layer confirmed the nano-porosity of the layers and give the size of the particles of about 10 nm for TiO₂ and 30 nm for WO₃ layer. Further analysis indicated that the titanium sol penetrates the WO₃ layer. Particles in the WO₃ layer consist of a crystalline monoclinic WO₃ core surrounded by a 5–10 nm amorphous phase consisting of WO₃, TiO₂ and SiO₂. The WO₃-TiO₂ layers were used to assemble all solid state photoelectrochromic (PE) devices. Under 1 sun irradiation (1000 W/m²) the visible transmittance of the PE device changes from 62% to 1.6%. The colouring and bleaching processes last about 10 minutes.     

Osmotic Coefficients of the Li<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript>+LiCl + H<Subscript>2</Subscript>O System at <Emphasis Type="Italic">T</Emphasis>=273.15 K

Osmotic coefficients and water activities for the Li₂B₄O₇+LiCl+H₂O system have been measured at T=273.15 K by the isopiestic method, using an improved apparatus. Two types of osmotic coefficients, φ _S and φ _E, were determined. φ _S is based on the stoichiometric molalities of the solute Li₂B₄O₇(aq), and φ _E is based on equilibrium molalities from consideration of the equilibrium speciation into H₃BO₃,B(OH)₄⁻ and B₃O₃(OH)₄⁻. The stoichiometric equilibrium constants K _m for the aqueous speciation reactions were estimated. Two types of representations of the osmotic coefficients for the Li₂B₄O₇+LiCl+H₂O system are presented with ion-interaction models based on Pitzer’s equations with minor modifications: model (I) represents the φ _S data with six parameters based on considering the ion-interactions between three ionic species of Li⁺, Cl⁻, and B₄O₇²⁻, and model (II) for represents the φ _E data based on considering the equilibrium speciation. The parameters of models (I) and (II) are presented. The standard deviations for the two models are 0.0152 and 0.0298, respectively. Model (I) was more satisfactory than model (II) for representing the isopiestic data.     

Thermal oxide synthesis and characterization of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanorods and Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> nanowires

Fe₃O₄ nanorods and Fe₂O₃ nanowires have been synthesized through a simple thermal oxide reaction of Fe with C₂H₂O₄ solution at 200–600°C for 1 h in the air. The morphology and structure of Fe₃O₄ nanorods and Fe₂O₃ nanowires were detected with powder X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The influence of temperature on the morphology development was experimentally investigated. The results show that the polycrystals Fe₃O₄ nanorods with cubic structure and the average diameter of 0.5–0.8 μm grow after reaction at 200–500°C for 1 h in the air. When the temperature was 600°C, the samples completely became Fe₂O₃ nanowires with hexagonal structure. It was found that C₂H₂O₄ molecules had a significant effect on the formation of Fe₃O₄ nanorods. A possible mechanism was also proposed to account for the growth of these Fe₃O₄ nanorods. Supported by the Fund of Weinan Teacher’s University (Grant No. 08YKZ008), the National Natural Science Foundation of China (Grant No. 20573072) and the Doctoral Fund of Ministry of Education of China (Grant No. 20060718010)     

Synthesis and characterization of La<Subscript>0.8</Subscript>Sr<Subscript>0.2</Subscript>Co<Subscript>0.5</Subscript>Fe<Subscript>0.5</Subscript>O<Subscript>3±δ</Subscript> nanopowders by microwave assisted sol–gel route

Nano-crystalline La_0.8Sr_0.2Co_0.5Fe_0.5O_3±δ powder has been successfully synthesized by microwave assisted sol–gel (MWSG) method. The decomposition and crystallization behavior of the gel-precursor was studied by Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD) analysis. From the result of FT-IR and X-ray diffraction patterns, it is found that a perovskite La_0.8Sr_0.2Co_0.5Fe_0.5O_3±δ was formed by irradiating the precursor at 700 W for 3 min, but the well-crystalline perovskite La_0.8Sr_0.2Co_0.5Fe_0.5O_3±δ was obtained at 700 W for 35 min. Morphological and specific area analysis of the powder were done by transmission electron microscopy (TEM), scanning electron microscope (SEM) and Brunauer–Emmett–Teller (BET). The surface areas measured was 38.9 m²/g and the grain size was ∼23 nm. Electrochemical properties of pure LSCF cathode on YSZ electrolyte at intermediate temperatures were investigated by using AC impedance analyzer, which shows a low area specific resistance (0.077 Ω cm² at 1073 K and 0.672 Ω cm² at 953 K). Moreover, the synthesis period of 20 h usually observed for conventional heating mode is reduced to a few minutes. Thus, the MWSG method is proved to be a novel, extremely facile, time-saving and energy-efficient route to synthesize LSCF powders.     

Microstructure and properties of sol-gel derived Pb(Zr<Subscript>0.3</Subscript>Ti<Subscript>0.7</Subscript>)O<Subscript>3</Subscript> thin films on GaN/sapphire for nanoelectromechanical systems

Highly (111) oriented, phase-pure perovskite Pb(Zr_0.3Ti_0.7)O₃ (or PZT 30/70) thin films were deposited on single-crystal, (0001) wurtzite GaN/sapphire substrates using the sol-gel process and rapid thermal annealing. The phase, crystallinity, and stoichiometry of annealed PZT films were evaluated by X-ray diffraction and Rutherford backscattering spectroscopy. The atomic force microscopy revealed a smooth PZT surface (rms roughness ∼1.5 nm) with striations and undulations possibly influenced by the nature of the underlying GaN surface. The cross-sectional field-emission scanning electron microscopic images indicated a sharper PZT/GaN interface compared to that of sol-gel derived PZT on (111) Pt/TiO₂/SiO₂/(100) Si substrates. The capacitance-voltage (C-V) characteristics for PZT in the Pt/PZT/GaN (metal-ferroelectric-semiconductor or MFS) configuration were evaluated as a function of annealing temperature and applied voltage. The observed C-V hysteresis stemmed from trapped charge at defect sites within PZT. Also, the lower capacitance density (C/A = 0.35 μF/cm², where A is the area of an electrode) and remnant polarization (P _r ∼ 4 μC/cm²) for PZT in the MFS configuration, compared to the values for PZT in the MFM configuration (Pt/PZT/Pt), were attributed to the high depolarization field within PZT.     

Low-temperature processing of sol-gel derived Pb(Zr,Ti)O<Subscript>3</Subscript> thick films using CO<Subscript>2</Subscript> laser annealing

Fabrication of ferroelectric Pb(Zr_0.52Ti_0.48)O₃ (PZT) thick films on a Pt/Ti/SiO₂/Si substrate using powder-mixing sol-gel spin coating and continuous wave CO₂ laser annealing technique to treat the specimens with at a relatively low temperature was investigated in the present work. PZT fine powders were prepared by drying and pyrolysis of sol-gel solutions and calcined at temperatures from 400 to 750°C. After fine powder-containing sol-gel solutions were spin-coated on a substrate and pyrolyzed, CO₂ laser annealing was carried out to heat treat the specimens. The results show that laser annealing provides an extremely efficient way to crystallize the materials, but an amorphous phase may also form in the case of overheating. Thicker films absorb laser energy more effectively and therefore melt at shorter periods, implying a significant volume effect. A film with thickness of 1 μm shows cracks and rough surface morphology and it was difficult to obtain acceptable electrical properties, indicating importance of controlling interfacial stress and choosing appropriate size of the mixing powders. On the other hand, a thick film of 5 μm annealed at 100 W/cm² for 15 s exhibits excellent properties (P _r = 36.1 μC/cm², E _c = 19.66 kV/cm). Films of 10 μm form a melting zone at the surface and a non-crystallized bottom layer easily at an energy density of 100 W/cm², showing poor electrical properties. Besides, porosity and electrical properties of thick films can be controlled using appropriate processing parameters, suggesting that CO₂ laser annealing of modified sol-gel films is suitable for fabricating films of low dielectric constants and high crystallinity.     

FTIR-ATR <Emphasis Type="Italic">in situ</Emphasis> observation on the efflorescence and deliquescence processes of Mg(NO<Subscript>3</Subscript>)<Subscript>2</Subscript> aerosols

The efflorescence and deliquescence processes of Mg(NO₃)₂ aerosol particles deposited on ZnSe substrate have been investigated through in situ Fourier transform infrared-attenuated total reflection (FTIR-ATR) technique at the molecular level. At relative humidity (RH) of ∼3%, Mg(NO₃)₂ particles existed as amorphous states. The amorphous Mg(NO₃)₂ particles were transformed into crystalline Mg(NO₃)₂ · nH₂O (n ≤ 5) with slight increasing of RH. Thermodynamically stable Mg(NO₃)₂·6H₂O crystals were gradually formed on the particle surface and started to be dissolved at the saturation point (∼53% RH). At the same time, a continuous phase transition from Mg(NO₃)₂ · nH₂O (n≤5) to Mg(NO₃)₂·6H₂O occurred on the particle surface. This led the solid particles to completely deliquesce at 76% RH, which was much higher than the saturation point of 53% RH. In the efflorescence process, Mg(NO₃)₂ droplets entered into the supersaturated region due to the gradual evaporation of water. Finally, amorphous particles were formed when RH decreased below 5%. In the FTIR-ATR spectra of the supersaturated Mg(NO₃)₂ droplets, the absorbance of the symmetric stretching vibration of NO ₃ ⁻ (v ₁- NO ₃ ⁻ ) clearly became stronger. It resulted from the continuous formation of solvent share ion pairs (SIPs), and even the contact ion pairs (CIPs) between Mg²⁺ and NO ₃ ⁻ . Supported by the Trans-Century Program Foundation for the Talents by the Ministry of Education of China, the National Natural Science Foundation of China (Grant Nos. 20073004, 20473012, and 20673010), the 111 Project (B07012), and the State Key Laboratory of Physical Chemistry for Solid Surface of Xiamen University     

Sol-gel processing of lead-free (Na,K)NbO<Subscript>3</Subscript> ferroelectric films

The sol-gel processing of lead-free (Na,K) NbO₃ ferroelectric films was studied. Sodium ethoxide (NaOC₂H₅) and potassium ethoxide (KOC₂H₅) were prepared by reacting solid Na and K with ethanol (99.7%) in a solvent of 2-methoxyethanol. 0.5-μm-thick (Na,K)NbO₃ thin films with orthorhombic perovskite structure were obtained by pyrolyzing at 400°C and annealing at 800–900°C. The films had relatively dense and uniform microstructure with grain size of about 50 nm, whose ferroelectricity was proved by the P-E hysteresis loop measurement. It was found that excess K was effective to reduce the annealing temperature for the crystallization of sol-gel-derived (Na,K)NbO₃ thin films.