Synthesis of nanocrystalline MgAl2O4 spinel powders by a novel chemical method

A novel chemical method for the preparation of nanocrystalline MgAl₂O₄ spinel powders has been developed in this paper. The mixed magnesium-aluminum hydroxide precipitates were initially formed in a three-dimensional space network microarea. After being calcined at above 700 °C, the nanocrystalline MgAl₂O₄ spinel powders were obtained. The precursor and as-calcined powders were characterized using TGA-DTA, XRD and TEM. The MgAl₂O₄ spinel powders calcined at 850 °C for 2 h are of narrow distribution, little agglomeration and small particle size of ∼ 24 nm. The reason for the synthesis of high-quality powders was explained.     

Synthesis of CaCu3Ti4O12 ceramic via a sol-gel method

The novel nano-ultrafine powders for the preparation of CaCu₃Ti₄O₁₂ ceramic were prepared by the sol-gel method and citrate auto-ignition method. The obtained precursor powders were pressed, sintered at 1000 °C to fabricate microcrystal CaCu₃Ti₄O₁₂ ceramic. The microcrystalline phase of CaCu₃Ti₄O₁₂ was confirmed by X-ray powder diffraction (XRD). The morphology and size of the grains of the powders and ceramics under different heat treatments were observed using scanning electron microscopy (SEM). The relative dielectric constant of the ceramic sintered at 1000 °C was measured with a magnitude of more than 10⁴ at room temperature, which was approaching to those of Pb-containing complex perovskite ceramics, and the loss tangent was less than 0.20 in a broad frequency region. The relative dielectric constant and loss tangent were also compared with that of CaCu₃Ti₄O₁₂ ceramic prepared by other reported methods.     

A co-precipitation technique to prepare BiTaO4 powders

A simple co-precipitation technique has been successfully used for the preparation of pure ultrafine single phase BiTaO₄. A standard ammonium hydroxide solution was used to precipitate Bi³⁺ and Ta⁵⁺ cations as hydroxides simultaneously under basic conditions. This precursor, on heating at 600 °C, produced product phase. This is the lowest temperature for the formation of BiTaO₄ phase so far reported in the literature. For comparison BiTaO₄ powders were also prepared by the traditional solid state method. The phase contents and lattice parameters were studied by the powder X-ray diffraction (XRD).     

Investigation and characterization of radio frequency sputtered Cr1.8Ti0.2O3−δ thin films derived by citrate, sol-gel and solid state routes

Cr_1.8Ti_0.2O_3−δ (CTO) powders were prepared by citrate, sol-gel and solid-state routes, respectively. The effect of preparation methods on the morphology of sputtered thin films was further investigated. X-ray diffraction patterns of the powders and films confirmed a single phase CTO. No impurity was observed even after sintering the powders at 1000 °C for 24 h. X-ray photoelectron spectroscopy analysis showed that Cr 2p (577.8 eV), and O 1 s (531.5 eV) core levels of the sputtered films have ∼1 eV variation in their binding energy positions compared to those of CTO powders. The atomic force microscopy analysis showed the grains of the films obtained by sputtering sol-gel powders had the smallest size in the range of 7-58 nm. The surface roughness of the thin films had the lowest value at 0.70 nm, whereas those obtained from solid state solution had the maximum value of 2.51 nm.     

Low-temperature acetone-assisted hydrothermal synthesis and characterization of BiFeO3 powders

Well-crystallized pure perovskite bismuth ferrite (BiFeO₃) powders have been synthesized by a facile hydrothermal route at the temperature as low as 130 °C with the aid of acetone. In the synthesis, acetone played important roles in the low-temperature synthesis of pure BiFeO₃. The as-prepared BiFeO₃ powders mainly consisted of cubic particles with the size range from 50 to 200 nm. zero-field-cooled and field-cooled magnetization measurements indicated that pure BiFeO₃ powders showed a spin-glass transition below the freezing temperature. The as-prepared pure BiFeO₃ powders showed weak ferromagnetism and ferroelectricity simultaneously at room temperature. Moreover, the bismuth ferrite BiFeO₃ exhibit efficient photocatalytic activity under visible light irradiation.     

Preparation of Ca-doped LaFeO3 nanopowders in a reverse microemulsion and their visible light photocatalytic activity

Nanoparticles of lanthanum ferrite (LaFeO₃) and calcium-doped LaFeO₃ (La_1−xCa_xFeO₃, x = 0.05-0.20) with perovskite-type structure have been prepared in a reverse microemulsion. Perovskite powder could be obtained at 500 °C for 3 h. The prepared powders were characterized by thermogravimetry-differential thermal analysis (TG-DTA), X-ray diffraction (XRD) and UV-Vis absorption spectroscopy. The visible-light photocatalytic activity of the photocatalysts was tested with methylene blue as an objective decomposition substance using fluorescence light as a visible light resource. The results showed that partial substitution of La³⁺ in LaFeO₃ with Ca²⁺ could decrease the crystalline size, enhance visible light absorption and improve visible light photocatalytic activity.     

The low temperature synthesis and characterization of Bi3.25La0.75Ti3O12 powder by hydroxide coprecipitation in aqueous medium

The fine powders of Bi_3.25La_0.75Ti₃O₁₂ (BLT) were prepared by coprecipitaton method in aqueous medium at low temperature. The differential thermal analysis (DTA), thermo-gravimetric analysis (TG) and X-ray diffraction (XRD) were employed to evaluate the phase formation of BLT and TEM was used to characterize and observe the particle size and morphology of BLT powder obtained. The results show that the bismuth layer perovskite phase of BLT can begin to form at as low as 500 °C by the coprecipitation method. When the precipitates obtained were calcined at 600 °C for 2 h, the mono-phase and perfect BLT powder was synthesized. The BLT powder obtained consists of irregular or plate-like particles which are less than about 100 nm and is nearly aggregate free.     

Growth of ultra-fine cobalt ferrite particles by a sol–gel method and their magnetic properties

Ultra-fine CoFe₂O₄ particles are fabricated by a sol–gel method and magnetic and structural properties of powders are investigated. Cobalt ferrite powders fired at and above 450 °C have only a single-phase spinel structure and behave ferrimagnetically. Powders annealed at 350 °C have a typical spinel structure and are of the paramagnetic and ferrimagnetic nature, simultaneously. With X-ray diffraction and Mossbauer spectroscopy measurements, the formation of nano-crystallized particles is confirmed when cobalt ferrite is annealed at 200 °C. In addition, the transition from the paramagnetic to the ferrimagnetic state is observed in samples fired at 200 °C as the measuring temperature decreases from the room to liquid nitrogen temperature. The magnetic behaviour of CoFe₂O₄ powders fired at and above 350 °C shows that an increase of the annealing temperature yields a decrease in the coercivity and, in contrast, an increase in the saturation magnetization. The maximum coercivity and the saturation magnetization of cobalt ferrite powders prepared by the sol–gel method are 2020 Oe and 76.5 e.m.u. g^–1, respectively.     

Gas sensor based on ordered mesoporous In2O3

We present the preparation of a semiconductor gas sensor based on ordered mesoporous In₂O₃. The In₂O₃ was synthesized by structure replication procedure from cubic KIT-6 silica. A detailed analysis of the morphology of the mesoporous powders as well as of the prepared sensing layer will be shown. Unique properties arise from the synthesis method of structure replication such as well defined porosity in the mesoporous regime and nanocrystallites with high thermal stability up to 450 °C. These properties are useful for the application in semiconducting gas sensors. Test measurements show sensitivity to methane gas in concentrations relevant for explosion prevention.     

Effects of addition of bismuth oxide and glass on the properties of lithium ferrite for ferrimagnetic pastes

Lithium ferrite, Li_0.5Fe_2.5O₄, has been prepared by decomposition of organometallic complexes at 800° C, and the optimization of heating schedule for conversion into ferrite has been studied. The effects of addition of glass, essential for adhesion of the ferrite film to alumina substrates, and bismuth oxide, as a sintering aid, on the properties and densification of lithium ferrite have been examined. X-ray diffraction, electron probe micro analysis (EPMA) and scanning electron microscopy (SEM) techniques have been used to study the solubility and distribution of bismuth oxide, grain growth and pore morphology. It has been found that the addition of bismuth oxide (up to 1.5 wt%) improves densification and increases resistivity of the lithium ferrite but the addition of glass causes a reduction of the resistivity. Although bismuth oxide forms a solid solution, it is not uniformly distributed throughout the ferrite phase. It is shown that the addition of bismuth oxide improves the insertion loss in microwave devices fabricated using ferrimagnetic pastes.     

Preparation and characterization of Co-doped TiO2 materials for solar light induced current and photocatalytic applications

Different amounts of Co-doped TiO₂ powders and thin films were prepared by following a conventional co-precipitation and sol–gel dip coating technique, respectively. The synthesized powders and thin films were subjected to thermal treatments from 400 to 800 °C and were thoroughly investigated by means of X-ray diffraction, X-ray photoelectron spectroscopy, energy dispersive analysis with X-rays, FT-infrared, FT-Raman, diffuse reflectance spectroscopy, ultraviolet–visible spectroscopy, BET surface area, zeta potential, flat band potential measurements, band-gap energy, etc. The photocatalytic ability of the powders was evaluated by methylene blue (MB) degradation studies. The thin films were characterized by photocurrent and ultraviolet–visible (UV–Vis) spectroscopy techniques. The characterization results suggest that the Co-doped TiO₂ powders synthesized in this study consist mainly anatase phase, and possess reasonably high specific surface area, low band gap energy and flat band potentials amenable to water oxidation in photoelectrochemical (PEC) cells. The photocatalytic degradation of MB over Co-doped TiO₂ powders followed the Langmuir–Hinshelwood first order reaction rate relationship. The 0.1 wt.% Co-doped TiO₂ composition provided the higher photocurrent, n-type semi-conducting behavior and higher photocatalytic activity among various Co-doped TiO₂ compositions and pure TiO₂ investigated.     

Mössbauer studies and magnetic properties of SnO2 doped with Fe

Transparent conducting SnO₂ powders doped with 10% Fe content were prepared by a polymerized complex method under acidic solutions, and annealed finally at 550 °C, and at 600 °C. These samples were characterized by X-ray diffraction, magnetization, and Mössbauer spectrometry at room temperature. Rutile SnO₂ phase was obtained for both samples, and the crystallite sizes were in the range of 13-14 nm. Both samples exhibit magnetization and the saturation magnetization was smaller for the sample annealed at 600 °C than for sample annealed at 550 °C. Room temperature Mössbauer spectra for both samples showed the presence of two different paramagnetic iron sites but no magnetic sextets. These results suggest that ferromagnetism originates from magnetic defects and not directly from iron ions.     

Comparison between titania thick films obtained through sol-gel and hydrothermal synthetic processes

Titania films were obtained through two synthetic processes, a traditional sol-gel method and a hydrothermal route. In SG synthesis, thermal decomposition of the precursor in oven at 400 °C for 2 h led to pure anatase TiO₂; in HY synthesis, instead, crystalline anatase grains were obtained in autoclave at 200 °C for 1 h. To investigate the microstructural evolution of SG and HY titania with temperature, each powder was annealed at 650, 750, 850 °C for 1 h and subjected to XRD analysis. Surprisingly, HY titania, contrary to SG, maintained the anatase phase, up to 850 °C, without any introduction of foreign elements. The sensing layers, obtained from as grown powders, were fired at 650, 750 or 850 °C and tested vs. methane and carbon monoxide. Both types of films fired at 850 °C yield insignificant responses to both CO and CH4, demonstrating the lack of influence of the crystalline phase on the gas response. Moreover, as regards the films fired at 650 and 750 °C, the gas responses are higher for SG than for HY samples, despite larger particle size.     

A low temperature route to prepare LaFeO3 and LaCoO3

A combination of digestion and further low temperature calcination to crystallize the product was employed to prepare LaFeO₃ (LF) and LaCoO₃ (LC) powders. Freshly co-precipitated lanthanum and ferric (or cobalt) hydroxide gels by sodium hydroxide were allowed to react at 100 °C under refluxing and stirring conditions for 4-6 h. These oven dried powders were heated at 450 °C to form crystalline LF (or LC) powders. The phase contents and lattice parameters were investigated by X-ray diffraction (XRD). Transmission electron microscope (TEM) investigations were carried out to examine the morphology and average particle size of these powders.     

Synthesis, characterization and dielectric properties of europium-doped barium titanate nanopowders

Europium-doped cubic barium titanate (BT) nanocrystals with % [Eu/Ti] mol ratio varying from 0.05 to 0.25 were prepared through hydrothermal route. The nano nature of these powders was confirmed by XRD and TEM studies. Pellets were prepared after calcining the powders at 1000 °C for 2 h. These pellets were annealed at 200, 500, 700 and 1000 °C for 2 h at each temperature and used for dielectric measurements. Raman spectra of two typical pellets with %[Eu/Ti] Eu/Ti mol ratios of 0.15 and 0.25 showed all the peaks characteristic of tetragonal BaTiO₃. Pure BT showed a low dielectric constant (DC) with a value of 398. Doping with small amounts of Eu resulted in many fold increase of DC values. A maximum value of 10576 at 1 KHz frequency was observed for the sample with % [Eu/Ti] mol ratio of 0.15. Lowering of Curie temperature T_c (95 to 110 °C) was observed for pure as well as Eu-doped barium titanate.     

Crystallinity and luminescent properties of citrate sol-gel derived BAM phosphor

By adopting a facile citrate sol-gel process, pure well-crystallized BaMgAl₁₀O₁₇:Eu²⁺(BAM) blue phosphors were successfully prepared. The crystallinity, particle size, morphology, chemical composition and luminescent properties were characterized by XRD, FE-SEM, EDS and spectrofluorometer respectively. The results indicated the precursor was amorphous below 800 °C, and begun to crystallize at 1100 °C. The synthesis temperature decreased by about 300 °C compared to solid-state method to prepare BAM. FE-SEM images showed a well-crystallized, platelet morphology with a particle size of 1-3 μm. The obtained BAM phosphors were fine powders and had higher luminescent intensity compared to solid-state derived BAM phosphors due to high purity and perfect crystalline morphology.     

Effect of processing temperature on characteristics of metal-ferroelectric (BiFeO3)-insulator (HfLaO)-silicon capacitors

The effect of temperature in rapid thermal annealing (RTA) process on the physical and electrical properties of bismuth ferrite ceramic thin films on HfLaO/p-Si substrates has been investigated. In metal-ferroelectric-insulator-silicon (MFIS) capacitors, the high-k HfLaO dielectric layer was prepared as the insulator layer. On HfLaO/Si substrates the bismuth ferrite thin film was fabricated via sputtering process with a BiFeO₃ (BFO) target at room temperature followed by RTA. The RTA temperature ranged from 500 to 700 °C. It is found that the root mean square roughness of ceramic films increases for high-temperature process. The maximum ferroelectric memory window is 1.6 V obtained from a sweep voltage of ± 4 V at the lowest RTA temperature of 500 °C. This good ferroelectric memory performance can be attributed to the low leakage current as a result of smooth surface of nanocrystalline ferroelectric BFO and Bi₂Fe₄O₉ thin films.     

Mechanically activated crystallization of phase pure nanocrystalline forsterite powders

Pure nanocrystalline forsterite was successfully synthesized via a mechanical activation route assisted with heat treatment. X-ray diffraction (XRD), scanning electron microscopy (SEM) and simultaneous thermal analysis (STA) techniques were utilized to characterize the prepared powders. Results showed that pure nanocrystalline forsterite could be fabricated completely by 10 h of mechanical activation and post-heat treatment at 1000-1200 °C for 1 h. By 10 h mechanical activation, the initial temperature of forsterite crystallization was reduced to about 820 °C. The obtained nanocrystalline forsterite powder had crystallite size about 57 nm according to Williamson-hall approach and particle sizes smaller than 1000 nm. XRD patterns showed that pure forsterite could not be obtained by mechanical activation process alone and that enstatite could be fabricated by increasing the time of milling. Liberation of CO₂ gas increased the rate of forsterite formation by increasing contact surface between grains.     

Luminescent properties of YAG:Ce phosphor powders prepared by hydrothermal-homogeneous precipitation method

Y_3 − xCe_xAl₅O₁₂ (YAG:Ce³⁺) phosphor powders were successfully prepared by hydrothermal-homogenous precipitation (HHP) method, under mild conditions with inexpensive aluminum and yttrium nitrates as the starting materials and urea as homogenous precipitant. The pure YAG crystalline phase could be formed after hydrothermal treatment at 100 °C for 4 h and 240 °C for 20 h and postannealing process at 1200 °C for 2 h. All of the as-prepared YAG:Ce³⁺ powders did not have the CeO₂ phase. The photoluminescence spectrum of crystalline YAG:Ce³⁺ phosphors showed the emission intensity of phosphor increased with increasing the annealed temperature and reached its maximum as the molar fraction of cerium ion was 0.10, and also showed the maximum emission wavelength nearly unchanged with the calcination temperature and cerium doping concentration.     

Preparation and characterization of new visible-light-driven BaCo0.5Nb0.5O3 photocatalyst

A new type of visible-light-driven photocatalyst BaCo_0.5Nb_0.5O₃ was successfully synthesized via a sol-gel process in this study. After heating the precursors at 1000 °C, a pure perovskite phase was obtained. The particle size and crystallinity of BaCo_0.5Nb_0.5O₃ powders markedly increased with a rise in the calcination temperature. The band gap of BaCo_0.5Nb_0.5O₃ calculated from the UV-visible spectra was found to be less than that of titania. BaCo_0.5Nb_0.5O₃ was demonstrated to have photocatalytic activity under visible light irradiation and this activity significantly depended on the synthesis temperature. The sol-gel derived powders were found to have better photocatalytic activity than the solid-state derived powders because of the reduced particle size and increased surface area.