Bio-template route for facile fabrication of Cd(OH)2@yeast hybrid microspheres and their subsequent conversion to mesoporous CdO hollow microspheres

Cadmium oxide (CdO) microspheres with a porous hollow microstructure were prepared by a facile yeast mediated bio-template route. The yeast provides a solid scaffold for the deposition of cadmium hydroxide (Cd(OH)₂) from cadmium acetate and sodium hydroxide solutions to form the hybrid Cd(OH)₂@yeast precursor. Thermal conversions of this at above 500 °C in air have produced hollow CdO microspheres. The products were characterized by scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FT-IR), thermal gravimetric and differential thermal analysis (TGA-DTA), and Brunauer-Emmett-Teller (BET) surface analysis respectively. The obtained CdO microspheres have uniform size (length = 2.6 ± 0.4 μm; width = 2.0 ± 0.2 μm) and a well defined, continuous, mesoporous hollow microstructure. The shell is about 250-280 nm in thickness. The mechanism of formation of Cd(OH)₂@yeast precursor and its conversion to CdO hollow microspheres is discussed. In comparison with traditional template-directed method, the present strategy represents a general, economical and environmentally benign route for the formation of metal oxide hollow microspheres. These materials have potential applications in different fields such as encapsulation, drug delivery, efficient catalysis, battery materials and photonic crystals. The method presented can be extended to the synthesis of other inorganic hollow microstructures of different sizes and shapes by pre-selecting suitable bio-templates.     

Fabrication of hollow mesoporous NiO hexagonal microspheres via hydrothermal process in ionic liquid

The novel NiO hexagonal hollow microspheres have been successfully prepared by annealing Ni(OH)₂, which was synthesized via an ionic liquid-assisted hydrothermal method. The samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), transmission electron microscopy (TEM), N₂ adsorption-desorption and Fourier transform infrared spectrometer (FTIR). The results show that the hollow NiO microstructures are self-organized by mesoporous cubic and hexagonal nanocrystals. The mesoporous structure possessed good thermal stability and high specific surface area (ca. 83 m²/g). The ionic liquid 1-butyl-3methylimidazolium tetrafluoroborate ([Bmim][BF₄]) was found to play a key role in controlling the morphology of NiO microstructures during the hydrothermal process. The special hollow mesoporous architectures will have potential applications in many fields, such as catalysts, absorbents, sensors, drug-delivery carriers, acoustic insulators and supercapacitors.     

Preparation of hollow hydroxyapatite microspheres by the conversion of borate glass at near room temperature

Hollow hydroxyapatite microspheres, consisting of a hollow core and a porous shell, were prepared by converting Li₂O-CaO-B₂O₃ glass microspheres in dilute phosphate solution at 37 °C. The results confirmed that Li₂O-CaO-B₂O₃ glass was transformed to hydroxyapatite without changing the external shape and dimension of the original glass object. Scanning electron microscopy images showed the shell wall of the microsphere was built from hydroxyapatite particles, and these particles spontaneously align with one another to form a porous sphere with an interior cavity. Increase in phosphate concentration resulted in an increase in the reaction rate, which in turn had an effect on shell wall structure of the hollow hydroxyapatite microsphere. For the Li₂O-CaO-B₂O₃ glass microspheres reacted in low-concentration K₂HPO₄ solution, lower reaction rate and a multilayered microstructure were observed. On the other hand, the glass microspheres reacted in higher phosphate solution converted more rapidly and produced a single hydroxyapatite layer. Furthermore, the mechanism of forming hydroxyapatite hollow microsphere was described.     

Synthesis and sintering of nanocrystalline hydroxyapatite powders by citric acid sol-gel combustion method

The citric acid sol-gel combustion method has been used for the synthesis of nanocrystalline hydroxyapatite (HAP) powder from calcium nitrate, diammonium hydrogen phosphate and citric acid. The phase composition of HAP powder was characterized by X-ray powder diffraction analysis (XRD). The morphology of HAP powder was observed by transmission electron microscope (TEM). The HAP powder has been sintered into microporous ceramic in air at 1200 °C with 3 h soaking time. The microstructure and phase composition of the resulting HAP ceramic were characterized by scanning electron microscope (SEM) and XRD, respectively. The physical characterization of open porosity and flexural strength have also been carried out.     

Synthesis reactions for Ti3SiC2 through pulse discharge sintering TiH2/Si/TiC powder mixture

Ternary compound Ti₃SiC₂ was rapidly synthesized by pulse discharge sintering the powder mixture of 1TiH₂/1Si/1.8TiC without preliminary dehydrogenation. Almost single-phase dense Ti₃SiC₂ was synthesized at 1400 °C for 20 min. The grain size of synthesized Ti₃SiC₂ strongly depends on sintering temperature. The synthesis mechanism of Ti₃SiC₂ was revealed to be completed via the reactions among the intermediate phases of Ti₅Si₃, TiSi₂ and the other reactants in the starting powder. The Ti-Si liquid reaction occurring above the Ti-Ti₅Si₃ eutectic temperature at 1330 °C was found to assist the synthesis reaction and densification of Ti₃SiC₂. The dehydrogenation of TiH₂ was accelerated by the synthesis reactions.     

Effect of dispersant on preparation of barium-strontium titanate powders through oxalate co-precipitation method

The quantitative precipitation of barium-strontium titanyl oxalate: (Ba_0.6Sr_0.4TiO(C₂O₄)₂·4H₂O, BSTO) precursor powders were successfully prepared through oxalate co-precipitation method. The pyrolysis of BSTO at 800 °C/4 h produced the barium-strontium titanate (Ba_0.6Sr_0.4TiO₃, BST) powders. Two kinds of dispersants namely ammonium salt of poly mathacrylic acid (PMAA-NH₄) and polyethylene glycol (PEG) were added respectively during the co-precipitation procedure. The powders were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), etc. Experimental results show that the addition of the dispersants reduced the productive rate of BST powders. The BSTO and BST powders obtained by aforementioned technique without dispersants were homogeneous with quasi-orbicular morphology. The particles grew into spindle shape with the effect of PEG. The morphology homogeneity was broke with small grains as well as large agglomerated particles concurrent when PMAA-NH₄ was added. The mechanism of the effect of the two dispersants was investigated in detail.     

A new and effective chemical reduction method for preparation of nanosized silver powder and colloid dispersion

Nanosized uniform silver powders and colloidal dispersions of silver were prepared from AgNO₃ by a chemical reduction method involving the intermediate preparation of Ag₂O colloidal dispersion in the presence of sodium dodecyle sulfate CH₃(CH₂)₁₁OSO₃Na as a surfactant. Several reducing agents such as hydrazine hydrate (N₂H₄·H₂O), formaldehyde (HCOH) and glucose (C₆H₁₀O₅) have been found to be preferable in this study from a practical point of view. The silver powder with the 60-120 nm particle size and colloidal dispersion with the particles size 10-20 nm and 0.5-2.0 wt.% concentration were successfully synthesized.     

Fabrication of Sr- and Co-doped lanthanum chromite interconnectors for SOFC

Many studies have been performed dealing with the processing conditions of electrodes and electrolytes in solid oxide fuel cells (SOFCs). However, the processing of the interconnector material has received less attention. Lanthanum chromite (LaCrO₃) is probably the most studied material as SOFCs interconnector. This paper deals with the rheology and casting behaviour of lanthanum chromite based materials to produce interconnectors for SOFCs. A powder with the composition La_0.80Sr_0.20Cr_0.92Co_0.08O₃ was obtained by combustion synthesis. Aqueous suspensions were prepared to solids loading ranging from 8 to 17.5 vol.%, using ammonium polyacrylate (PAA) as dispersant and tetramethylammonium hydroxide (TMAH) to assure a basic pH and providing stabilization. The influence of the additives concentrations and suspension ball milling time were studied. Suspensions prepared with 24 h ball milling, with 3 wt.% and 1 wt.% of PAA and TMAH, respectively, yielded the best conditions for successful slip casting. Sintering of the green discs was performed in air at 1600 °C for 4 h leading to relatively dense materials.     

Synthesis, crystal structure, dielectric properties, and potential use of nanocrystalline complex perovskite ceramic oxide Ba2ErZrO5.5

A new member belongs to Ba₂REZrO_5.5 (RE = Rare-Earth) perovskites, viz. Ba₂ErZrO_5.5, is synthesized as nanocrystals using a combustion process. Unlike the other Ba₂REZrO_5.5 perovskites, which are cubic, Ba₂ErZrO_5.5 crystallizes in tetragonal structure having space group P4/mnc (#128). Phase purity and ultrafine morphology of Ba₂ErZrO_5.5 powders were examined using X-ray diffraction (XRD), differential thermal analysis (DTA), thermogravimetric analysis (TGA), Fourier transform of infrared spectroscopy (FT-IR), Brunauer-Emmett-Teller (BET) specific surface area measurements, and transmission electron microscopy (TEM). Nanocrystals of Ba₂ErZrO_5.5 was sintered at ∼1500 °C for 4 h; whereas coarse-grained powders synthesized through solid state reaction could not be sintered even at 1700 °C for prolonged duration. XRD pattern of Ba₂ErZrO_5.5 was refined for atomic coordinates, lattice parameters, occupancies, and thermal factors using Rietveld analysis of XRD. Dielectric constant (?′) of Ba₂ErZrO_5.5 at 10 MHz is 21.62 ± 2 and dielectric loss (tan δ) is 5 × 10⁻³ at room temperature. Similar to the other Ba₂REZrO_5.5 perovskites, Ba₂ErZrO_5.5 is also showed chemically stability with YBa₂Cu₃O_7−δ (YBCO) and Bi₂Sr₂Ca₁Cu₂O_x (Bi-2212) superconductors. Ba₂ErZrO_5.5 can possibly be used as a substrate for high temperature superconductor (HTS) films, or be used as an insulator in the active superconductor-insulator-superconductor (SIS) structures.     

The crystallinity of calcium phosphate powders influenced by the conditions of neutralized procedure with citric acid additions

Calcium phosphate powders with nano-sized crystallinity were synthesized by neutralization using calcium hydroxide and orthophosphoric acid with the assistance of citric acid. The influence of processing parameters, such as free or additive citric acid, synthetic temperature and ripening time, on the crystallinity of hydroxyapatite were investigated. The results of X-ray diffraction and microstructure observations showed that the crystallinity and morphology of nano-sized hydroxyapatite particles were influenced by the presence or absence of citric acid. It was found that the crystallinities and crystallite sizes of hydroxyapatite powders prepared with the additive citric acid increased with increasing synthetic temperature and ripening time. Especially, the crystallinities of (h k 0) planes were raised and more homogeneously grown particles were obtained with increasing synthetic temperature.     

Fabrication and characterization of mesoporous TiO2/polypyrrole-based nanocomposite for electrorheological fluid

Mesoporous TiO₂/polypyrrole (PPy)-based nanocomposite for electrorheological fluid was synthesized through one-pot method. By exploiting the combination conductivity of PPy and high dielectric constant of TiO₂, the ER fluid exhibited an enhanced effect. The shear stress was 3.3 times as high as that of mesoporous TiO₂. Powder X-ray diffraction (XRD), TEM and Fourier transform infrared (FT-IR) spectroscopy were employed to characterize the as-made samples. Using a modified rotational viscometer, the electrorheological effect was measured. Dielectric spectra were also given to explain the mechanism.     

Adsorption of vitamin E on mesoporous titania nanocrystals

Tri-block nonionic surfactant and titanium chloride were used as starting materials for the synthesis of mesoporous titania nanocrystallite powders. The main objective of the present study was to examine the synthesis of mesoporous titania nanocrystals and the adsorption of vitamin E on those nanocrystals using X-ray diffraction (XRD), transmission electron microscopy, and nitrogen adsorption and desorption isotherms. When the calcination temperature was increased to 300 °C, the reflection peaks in the XRD pattern indicated the presence of an anatase phase. The crystallinity of the nanocrystallites increased from 80% to 98.6% with increasing calcination temperature from 465 °C to 500 °C. The N₂ adsorption data and XRD data taken after vitamin E adsorption revealed that the vitamin E molecules were adsorbed in the mesopores of the titania nanocrystals.     

Si-substituted hydroxyapatite nanopowders: Synthesis, thermal stability and sinterability

Synthetic hydroxyapatites incorporating small amounts of Si have shown improved biological performances in terms of enhanced bone apposition, bone in-growth and cell-mediated degradation.This paper reports a systematic investigation on Si-substituted hydroxyapatite (Si 1.40 wt%) nanopowders produced following two different conventional wet methodologies: (a) precipitation of Ca(NO₃)₂·4H₂O and (b) titration of Ca(OH)₂. The influence of the synthesis process on composition, thermal behaviour and sinterability of the resulting nanopowders is studied.Samples were characterised by electron microscopy, induced coupled plasma atomic emission spectroscopy, thermal analysis, infrared spectroscopy, N₂ adsorption measurements, X-ray diffraction and dilatometry. Semicrystalline Si-substituted hydroxyapatite powders made up of needle-like nanoparticles were obtained, the specific surface area ranged between 84 and 110 m²/g. Pure and Si-substituted hydroxyapatite nanopowders derived from Ca(NO₃)₂·4H₂O decomposed around 1000 °C. Si-substituted hydroxyapatite nanopowders obtained from Ca(OH)₂ were thermally stable up to 1200 °C and showed a distinct decreased thermal stability with respect to the homologous pure sample. Si-substituted hydroxyapatites exhibited higher sintering temperature and increased total shrinkage with respect to pure powders. Nanostructured dense ceramics were obtained by sintering at 1100 °C Si-substituted hydroxyapatites derived from Ca(OH)₂.     

Preparation of SrCe0.95Yb0.05O3−α perovskite for use as a membrane material in hollow fibre fabrication

In this study, a pure orthorhombic perovskite SrCe_0.95Yb_0.05O_3−α (SCYb) ceramic powder was prepared using a chelated water-soluble complex method at relatively low temperatures. Common chelating ligands such as citric acid and EDTA were employed for the synthesis of complex-based precursors, followed by thermal decomposition of the precursors at high temperatures. Thermal behaviour, crystal phases, and structures of the prepared ceramic powders were characterised by TGA/DTA, XRD and SEM techniques, respectively. Clean and single-phase SCYb submicron-sized powders were obtained after sintering at a temperature of 1000 °C for 4 h. Gas-tight hollow fibre having a maximum bending strength of 60 MPa have been fabricated from the SCYb powders synthesised from the chelated water-soluble complex method.     

Combustion of TiO2-Mg and TiO2-Mg-C systems in the presence of NaCl to synthesize nanocrystalline Ti and TiC powders

The combustion process of TiO₂-Mg and TiO₂-Mg-C systems with sodium chloride as an inert diluent was investigated. The values of combustion parameters and temperature distribution on a high-temperature wave according to the amount of sodium chloride were obtained by the thermocoupling technique. The leading stages of combustion processes are found and the sizes of reactionary zones were estimated. It is shown that the introduction of NaCl in an initial mixture promotes the formation of a nanocrystalline structure of the final products. As a result, nanosized titanium, and titanium carbide powders have been successfully obtained.     

Microwave-hydrothermal process for the synthesis of rutile

The synthesis of pure rutile titanium dioxide is not an easy achievement, as the crystallization process generally leads to mixtures of two or even three phases; moreover the synthetic processes normally used by industry require harsh reaction conditions. We carried out the synthesis of titanium dioxide from an aqueous titanium tetrachloride solution under microwave irradiation in the reaction time range of 5-120 min. We mostly obtained mixtures of rutile and anatase, but obtained single-phase rutile after a 2-h treatment at 160 °C; transmission electron micrographs revealed well-dispersed spherical nanoparticles. We also investigated the effects of dilution and addition of a dispersant (polyvinylpyrrolidone) on phase crystallization and particle shape.     

Ammonium hydrogen difluoride induced fluorination of hydroxyapatite

Ammonium hydrogen difluoride (NH₄HF₂) was used as a fluorinating agent for the fluorination of hydroxyapatite. The process was carried out at room temperature by pH cycle method, at 80 °C by reflux method and at 900 °C by solid state reaction. The products obtained in all the three methods were analyzed by powder XRD and FT-IR techniques to confirm fluorination. FT-IR spectra showed the decrease in the intensity of ‘OH’ stretching band at 3571 cm⁻¹ and complete disappearance of ‘OH’ bending band at 632 cm⁻¹. Appearance of a new peak at around 740 cm⁻¹ (due to the formation of .H-F. bond) was observed in all the spectra. Similarly, powder XRD patterns exhibited higher angle shift in 2θ values of high intense peaks associated with (300), (112) and (211) planes when compared to parent hydroxyapatite. Both the structural and spectroscopic analyses confirmed partial fluorination of hydroxyapatite.     

Fabrication of hollow silver spheres by MPTMS-functionalized hollow silica spheres as templates

In this study, we provide a strategy to prepare the hollow silver spheres by accumulating the silver nanoparticles on the surface of 3-mercaptopropyltrimethoxysilane (MPTMS)-functionalized silica as templates, which was accomplished by the chemisorption between silver nanoparticles and thiol groups. Then, the resulting hollow silver spheres were obtained through the chemical wet etching process with 10 M HF solution. In conventional method, the fabrication of hollow silver spheres from core-shell spheres was not easy due to the difficulties in retaining the shell structures during core removal. The method in this paper could overcome this limitation. The major focus of study is on understanding the mechanism of formation of the hollow silver spheres through the self-assembly behavior by chemisorption between silver nanoparticles and thiol groups. The silver-coated silica and hollow silver spheres were characterized by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), high-resolution TEM (HR-TEM), and X-ray photoelectron spectroscopy (XPS).     

Densification of nanocrystalline ITO powders in fast firing: effect of specimen mass and sintering atmosphere

Nano-sized indium tin oxide (ITO) powders were prepared by a coprecipitation method, and the sintering characteristics in fast firing were examined. The mass of the specimen, sintering atmosphere and sintering temperature varied. Oxygen atmosphere promoted the densification in normal rate sintering, while oxygen inhibited the densification in fast firing. Fast firing severely retarded densification as the mass of the specimen and the sintering temperature increased. This was explained by differential densification, which could easily occur in conditions with a high densification rate and a high thermal gradient in the specimen, where the outer region of the specimen densifies much faster than the center. Once the highly densified outer skeleton is formed, the inside of the specimen is difficult to densify because the outer skeleton geometrically constrains densification.     

Efficient rapid microwave-assisted route to synthesize InP micrometer hollow spheres

The efficiencies of two methods of synthesizing InP micro-scale hollow spheres are compared via the analogous solution-liquid-solid (ASLS) growth mechanism, either through a traditional solvothermal procedure, or via a microwave-assisted method. Scanning electronic microscopy (SEM) images show that most of the as-grown samples are micrometer hollow spheres, which indicates the efficiency of both methods. For traditional solvothermal route, long time (10 h) is necessary to obtain the desired samples, however, for the microwave-assisted route, 30 min is enough for hollow spherical products. An optimal choice of microwave irradiating time allows reducing the reaction time from hours to minutes. The proposed ASLS growth mechanism has also been discussed in detail.