Stability of tetragonal ZrO2 phase in Al2O3 prepared from Zr-Al organometallic compounds

Zr-Al organometallic compounds have been spray-dried and heated at temperatures 600 to 1400°C to prepare ZrO₂-Al₂O₃ composite powders. The powders consist of balloon-like particles 0.5 to 2 m in diameter with homogeneously dispersed tetragonal ZrO₂ grains 0.1 to 0.2 m in diameter. The tetragonal fraction of ZrO₂ in the composite powders is higher than that in the powders prepared from sols of Zr(OBuⁿ)₄ and Al[OCH(CH₃)₂]₃. The fraction is affected by the organofunctional group in the Zr-Al compounds.Zr(OBuⁿ)₄ = Zr(OC₄Hgⁿ)₄; Al[OCH(CH₃)₂]₃ = Al(OPrⁱ)₃.     

Water-based sol–gel nanocrystalline barium titanate: controlling the crystal structure and phase transformation by Ba:Ti atomic ratio

Highly stable, water-based barium titanate (BaTiO₃) sols were developed by a low cost and straightforward sol–gel process. Nanocrystalline barium titanate thin films and powders with various Ba:Ti atomic ratios were produced from the aqueous sols. The prepared sols had a narrow particle size distribution in the range 21–23 nm and they were stable over 5 months. X-ray diffraction pattern revealed that powders contained mixture of hexagonal- or perovskite-BaTiO₃ as well as a trace of Ba₂Ti₁₃O₂₂ and Ba₄Ti₂O₂₇ phases, depending on annealing temperature and Ba:Ti atomic ratio. Highly pure barium titanate with cubic perovskite structure achieved with Ba:Ti = 50:50 atomic ratio at the high temperature of 800 °C, whereas pure barium titanate with hexagonal structure obtained for the same atomic ratio at the low temperature of 500 °C. Transmission electron microscope revealed that the crystallite size of both hexagonal- and perovskite-BaTiO₃ phases reduced with increasing the Ba:Ti atomic ratio, being in the range 2–3 nm. Scanning electron microscope analysis revealed that the average grain size of barium titanate thin films decreased with an increase in the Ba:Ti atomic ratio, being in the range 28–35 nm. Moreover, based on atomic force microscope images, BaTiO₃ thin films had a columnar-like morphology with high roughness. One of the highest specific surface area reported in the literature was obtained for annealed powders at 550 °C in the range 257–353 m²g⁻¹.     

Synthesis of single-crystalline perovskite barium titanate nanorods by a combined route based on sol-gel and surfactant-templated methods

The single-crystalline perovskite barium titanate nanorods were successfully synthesized by a combined route based on sol-gel and surfactant-templated methods at low temperature. The synthesis was accomplished by using barium acetate (Ba(CH₃COO)₂) and tetrabutyl titanate (Ti[O(CH₂)₃CH₃]₄) as the starting materials and laurylamine as the surfactant, respectively. Well-isolated single-crystalline cubic perovskite BaTiO₃ nanorods with diameters ranging from 20 to 80 nm and lengths reaching up to > 10 μm can be easily fabricated by this route. The crystal form and morphology of the nanorods were characterized by X-ray diffraction, TEM and HRTEM. The mechanism of the formation of the single-crystalline cubic perovskite BaTiO₃ nanorods was discussed based on the theory of oriented attachment.     

Microstructural characteristics and evolution of Ti2AlN/TiAl composites with a network reinforcement architecture during reaction hot pressing process

The microstructural evolution of TiAl matrix composites with a novel network distribution of Ti₂AlN particle reinforcement was studied. The composites were synthesized by reaction hot pressing method using pure Al and nitrided Ti powders as initial materials. Pure Ti powders nitrided at 600 °C for a certain time in an atmosphere of flowing nitrogen turned into new compound Ti(N) powders, which have a shell of titanium nitrides (such as TiN, Ti₂N and TiN_0.3) and a core of Ti–N solid solution. Within the composites synthesized, Ti₂AlN particles, produced by in situ reaction, exhibit a network distribution. The special shell/core structure of the compound Ti(N) powders contributes to this architecture. Nitriding time of the Ti powders greatly affects the microstructure of the composites. Increasing the nitriding time is beneficial to the distribution of Ti₂AlN particles in a continuous network form. However, too long nitriding time can result in the aggregation of Ti₂AlN particles and thus destroy the uniformity of the network structure. The in-situ synthesized Ti₂AlN/TiAl composites with uniform network structure have a superior mechanical property, and their compressive strengths at 800 °C and 1000 °C are 1112 MPa and 687 MPa, respectively.     

Synthesis and characteristics of near-stoichiometric barium titanate powder by low temperature hydrothermal reaction using titanium tetra(methoxyethoxide)

Near-stoichiometric BaTiO₃ powders with ultrafine particle size and high crystallinity were prepared by low temperature hydrothermal reaction of Ba(OAc)₂ and Ti(OCH₂CH₂OCH₃)₄. BaTiO₃ particles were synthesized in the spherical, metastable cubic crystalline grains with size distribution between 60–90 nm in diameter. Ultrafine particle size was resulted from the control of the hydration rate and the decrease of Ti-O-Ti cross-linking extent of titanium precursor, Ti(OCH₂CH₂OCH₃)₄, which gives electronic, steric, and weakly chelating effect to titanium ion. Increasing the Ba/Ti mole ratio in reactant could not overcome the notorious Ba-deficiency but, improved stoichiometry and produced finer and less agglomerated particles. Interestingly, adding a slight pressure to autogeneous hydrothermal condition (total 4–10 atm) has yielded near-stoichiometric, highly crystalline, and less agglomerated BaTiO₃ particles. These particles, which were in metastable cubic form as synthesized, initiated phasetransition to tetragonal form by calcination at below 400 °C.     

Growth and characterization of well-aligned rutile TiO2 nanocrystals on sapphire substrates via metal organic vapour deposition

Well-aligned TiO₂ nanocrystals (NCs) were prepared by metalorganic chemical vapour deposition on sapphire (SA) (100) and (012) substrates, using Ti[OCH(CH₃)₂]₄ as precursor. The surface morphology, structural and spectroscopic properties of the deposited NCs were characterized using field emission scanning electron microscopy (FESEM), X-ray diffractometry (XRD) and Raman spectroscopy (RS). FESEM micrographs show that the NCs on SA(100) are vertically aligned. XRD determines the preferable orientations of the as-deposited NCs. RS confirms rutile phase of the NCs. The roles of substrate orientations for the formation of different textures of TiO₂ NCs are discussed.     

The effect of solution chemistry on the preparation of MgAl2O4 by hydrothermal-assisted sol-gel processing

Preparation of magnesium aluminate spinel powder by hydrothermal-assisted sol-gel processing from MgAl₂(OCH₂CH₂OR)₈, RCH₃ (1), CH₂CH₂OCH₃ (2), MgAl₂[OCH(CH₃)₂]₈ (3) and MgAl₂(O-^sBu)₈ (4) in toluene and parent alcohol has been investigated. Coordination status of aluminum atom in precursors was determined by ²⁷Al NMR and correlation between coordination number of aluminum and development of spinel phase in hydrothermal-assisted sol-gel processing has been studied. The gels obtained from hydrothermal-assisted hydrolysis of magnesium-aluminum alkoxides that contain six-coordinated aluminum atoms in solution (1 and 2) after calcination at 700 °C resulted in the formation of pure spinel phase, whereas in similar hydrolysis and calcination processes of precursors that contain four-coordinated aluminum (3 and 4) spinel phase forms along with some Al₂O₃ and MgO. Selected powders obtained from hydrothermal-assisted sol-gel processing were characterized by thermal analysis (TGA/DSC), X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). Results indicate that the coordination status of aluminum in the precursor is very crucial for the formation of pure phase spinel. The morphology of prepared spinels was studied by SEM and the results showed that the solvent in hydrothermal-assisted sol-gel processing has a marked effect on the morphology of the resulting MgAl₂O₄. In hydrothermal-assisted sol-gel processing of aluminum-magnesium alkoxides in hydrophobic solvent, spherical particles are formed, while in the parent alcohol, non-spherical powders are formed.     

X-ray diffraction and microstructural studies on hydrothermally synthesized cubic barium titanate from TiO2-Ba(OH)2-H2O system

In our earlier studies synthesis of pure barium titanate (BT) from TiO₂-Ba(OH)₂-NH₃ system was reported. This work describes a novel route for preparing cubic barium titanate from TiO₂-Ba(OH)₂-H₂O system without addition of any mineraliser. The experimental parameters varied were: reaction time (half-an-hour to 3 h), reaction temperature (80 to 150 °C) and [Ba/Ti] ratio (0.92 to 1.64). The progress of reaction for formation of BT was monitored by analyzing the X-ray diffraction data obtained under different processing conditions. Mono-phasic barium titanate powder having a mean crystallite diameter (MCD) of 26 nm along (101) plane was formed when the reaction was carried out for 3 h at 150 °C. The estimated strains on the planes of the BT nano-crystals were found to be negligible. The microstructure of pure BT showed the particles to be of single crystallite nature with average size matching with the MCD value calculated from the XRD data.     

Hydrothermal-polyol route to synthesis of β-Ni(OH)2 and NiO in mixed solvents of 1,4-butanediol and water

The β-Ni(OH)₂ with flower-like morphology assembled from nanosheets has been successfully synthesized by a hydrothermal-polyol method from Ni(CH₃COO)₂·4H₂O in mixed solvents of 1,4-butanediol and water at 200 °C for 24 h. The NiO with similar morphology was obtained by a simple thermal decomposition of the precursor (β-Ni(OH)₂) at 400 °C for 3 h in air. The products were characterized by X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), thermogravimetric analysis (TG) and differential scanning calorimetric analysis (DSC). We expect that this hydrothermal-polyol method may be extended to the preparation of nanostructures of other kinds of metal oxides.     

Surface structure and visible light photocatalytic activity of titanium–calcium hydroxyapatite modified with Cr(III)

The synthetic titanium–calcium hydroxyapatite (Ti–CaHap) particles were treated with different concentrations of aqueous Cr(NO₃)₃·9H₂O solution and the materials obtained were characterized by a variety of conventional techniques. The crystal structure and particle morphology of Ti–CaHap were essentially not altered by treating with Cr(III) solution. With increasing the Cr(III) concentration, the amount of Cr(III) in the products was increased and that of Ca(II) was decreased. XPS results revealed that the surface state of Cr of Ti–CaHap was trivalent. These facts allow us to infer that the Cr(III) was doped by substitution of surface Ca(II) of Ti–CaHap. Besides, IR results proved that increasing the Cr(III) concentration developed the surface Cr–OH band while the surface Ti–OH and P–OH bands of Ti–CaHap vanished. This imply that the formation of surface P–O-Cr(OH)₂ and Ti–O–Cr(OH)₂ groups, resulting the Cr(OH)₃-like layer on the surface of Ti–CaHap particles. The Cr(III)-doped Ti–CaHap possessed the absorption peaks at 446 and 623 nm in vis range in addition to the UV absorption of charge transfer transition of O^2? → Ti⁴⁺. The vis absorption peaks developed on raising the Cr(III) concentration. The photocatalytic decomposition of acetaldehyde into CO₂ over Cr(III)-doped Ti–CaHap was detected under vis irradiation and the activity was lowered by the formation of Cr(OH)₃-like layer on the particle surface.     

Preparation of barium titanate nanocube particles by solvothermal method and their characterization

Barium titanate (BaTiO₃) nanocube particles below 20 nm were prepared by solvothermal method. A selection of organic solvent and inorganic materials of Ba and Ti sources was most important for the preparation of nanocubes. A nucleation and particle growth of BaTiO₃ nanoparticles led to a formation of the BaTiO₃ nanocubes with a size of 10–15 nm at temperatures above 200 °C.     

Sol–gel synthesis of vanadium pentoxide nanoparticles in air- and water-stable ionic liquids

Vanadium pentoxide (V₂O₅) nanoparticles were synthesized at moderate reaction temperatures by hydrolysis of VO[OCH(CH₃)₂]₃ in two different air- and water-stable ionic liquids with the same anion: 1-butyl-1-methyl pyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py_1,4]Tf₂N) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([EMIM]Tf₂N) via the sol–gel method using acetone and isopropanol either as refluxing solvents or as co-solvents. The cation type of the ionic liquid affects the crystallinity, morphology, and surface area of the produced nanoparticles: [Py_1,4]Tf₂N gave products with higher crystallinity especially with acetone as a refluxing and co-solvent, while [EMIM]Tf₂N gave a clear mesoporous morphology with isopropanol as a refluxing solvent. Ionic liquids affect the key factors (morphology and surface area) that make V₂O₅ an attractive material as catalyst and/or cathodic material for lithium ion batteries.     

Crystallization behavior of nano-ceria powders by hydrothermal synthesis using a mixture of H2O2 and NH4OH

Homogeneous and well-crystallized nano-ceria (CeO₂) powders were produced by hydrothermal synthesis using a mixture of H₂O₂ as the oxidizer and NH₄OH as the mineralizer. The precipitates were prepared by mixture ligands of OH and OOH derived from NH₄OH and H₂O₂. The hydrothermal synthesis was conducted at 200 °C for 6 h.The synthesized ceria powders were characterized as the crystalline phase identification by X-ray diffractometry, the chemical bonding by infrared spectrophotometer (FT-IR) and the morphology and particle size by transmission electron microscopy (TEM).Crystallization behavior of ceria powders using a mixture of oxidizer and mineralizer was more influenced by OOH ligands than by OH ligands to form Ce(OH)_x(OOH)_4−x as precursor with increasing concentrations of H₂O₂ with 10–20 times of the cerium(III) ion. In all samples, synthesized particle size was decreased with increasing concentration of oxidizer ranging from 6 to 12 nm in diameter. Morphology and size distribution of synthesized particles were relatively spherical and uniform, respectively.     

Effects of Ti and a twice-quenching treatment on the microstructure and ductile brittle transition temperature of 9CrWVTiN steels

The effects of Ti and a twice-quenching treatment on the microstructure and ductile brittle transition temperature (DBTT) of 9CrWVTiN steels have been studied. The results show that Ti addition reduces austenite grain size and martensitic lath, and moderate Ti (< 0.018%) content reduces the precipitates size. Microstructure, especially the coarse M₂₃C₆ precipitates is remarkably refined by twice quenching and by consuming C through preferential precipitation of MX precipitates in the furnace cooling period, thus contributing to the decrease of DBTT compared with quenching–tempering process. However, DBTT increases with increasing Ti content, and the increased DBTT reaches to as high as 36 °C compared with steel without Ti regardless of the refinement of microstructure. Through precipitate analyses, we find that, Ti strongly interferes with the precipitation of V(C,N). Ti(C,N) hardly exists in matrix alone. Instead, it acts as the core of quadrate (Ti,V)(C,N) particle. Big stress concentrations at the corners of coarse quadrate (Ti,V)(C,N) precipitates make them as crack initiators during impact tests, thus deteriorating the toughness. Also, formation of complex (Ti,V)(C,N) particles reduces the amounts of V, N, and C available for adequate fine V(C,N) particle precipitation. These two factors are the main reasons for high DBTT generated by Ti addition.     

Ceramic/natural rubber composites: influence types of rubber and ceramic materials on curing,mechanical, morphological,and dielectric properties

Influence of different types of rubber and ceramic material on cure characteristics, mechanical, morphological, and dielectric properties of natural rubber (NR) vulcanizate was studied. Two types of ferroelectric ceramic materials: barium titanate (BaTiO₃) and lead titanate (PbTiO₃) were prepared by solid-state reaction with calcinations at 1100 °C for 2 h. The ceramic powders were then characterized by X-ray diffraction (XRD), particle size analyzer, and SEM techniques. Ceramic/rubber composites were then prepared by melt mixing of rubber and ceramic powders. Two different types of NR (i.e., epoxidized NR [ENR] and unmodified NR) and two types of ceramic powders (i.e., BaTiO₃ and PbTiO₃) were exploited. It was found that incorporation of ceramic powders in rubber matrix and the presence of epoxirane rings in ENR molecules caused faster curing reaction, and higher delta torque but lower elongation at break. This is attributed to lower mobility of molecular chains and higher interaction between ENR molecules. Furthermore, SEM results revealed that the BaTiO₃ composites showed finer and better distribution of the particles in the rubber matrix than that of the PbTiO₃ composite. This caused superior mechanical properties of the BaTiO₃ composites. Furthermore, higher dielectric constant and loss tangent was observed in the ENR/BaTiO₃ composites.     

Preparation and properties of fluorinated aliphatic alcohols/silica nanocomposites – Application to the encapsulation of anatase titanium oxide nanoparticles into these composite cores

A variety of fluorinated aliphatic alcohols [R_F-CH₂CH₂OH; R_F = CF₃(CF₂)₃CH₂(CF₂)₅, CF₃(CF₂)_n; n = 3, 5, 7] were applied to the preparation of the corresponding fluorinated alcohols/silica nanocomposites [R_F-CH₂CH₂OH/SiO₂] through the sol–gel reactions with tetraethoxysilane (TEOS) and silica nanoparticles under alkaline conditions. R_F-CH₂CH₂OH/SiO₂ nanocomposites thus obtained have a good dispersibility and stability in not only water but also traditional organic solvents such as methanol, ethanol, 1,2-dichloroethane and tetrahydrofuran. FE–SEM (field emission scanning electron microscopy) and dynamic light scattering (DLS) measurements show that these composites are nanometer size-controlled fine particles in methanol. These fluorinated nanocomposites were also applied to the surface modification of glass to exhibit a superhydrophilic characteristic with a superoleophobicity on the modified surface. Interestingly, R_F-CH₂CH₂OH/SiO₂ nanocomposites were found to exhibit no weight loss behavior corresponding to the contents of the alcohols in the composites even after calcination at 800 °C. In addition, anatase TiO₂ nanoparticles (an-TiO₂) were effectively encapsulated into R_F-CH₂CH₂OH/SiO₂ nanocomposite cores under the similar sol–gel reactions to give the corresponding fluorinated alcohol/SiO₂/an-TiO₂ nanocomposites. These obtained nanocomposites can give a higher photocatalytic activity even after calcination at 1000 °C for the decolorization of methylene blue under UV light irradiation than that of the an-TiO₂ nanoparticles under the similar conditions, although the parent TiO₂ nanoparticles after calcination were unable to give a photocatalytic activity.     

Biosynthesis of schwertmannite by Acidithiobacillus ferrooxidans cell suspensions under different pH condition

Oxidation of FeSO₄ solution with initial pH in the range of 1.40–3.51 by Acidithiobacillus ferrooxidans LX5 cell at 26 °C and subsequent precipitation of resulting Fe(III) were investigated in the present study. Results showed that the oxidation rate of Fe(II) was around 1.2–3.9 mmol l^? 1 h^? 1. X-ray diffraction (XRD) indicated that the formed precipitates were composed of natrojarosite with schwertmannite when the initial pH was 3.51, while only schwertmannite was produced when initial pH was in the range of 1.60–3.44 and no precipitate occurred when initial pH ≤ 1.40. Scanning electron microscope (SEM) analyses showed that precipitates formed in solution with initial pH 3.51 were spherical particles of about 0.4 µm in diameter and had a smooth surface, whereas precipitates in solution with initial pH ≤ 3.44 were spherical particles of approximately 1.0 µm in diameter, having specific sea-urchin morphology. Specific surface area of the precipitates varied from 3.42 to 23.45 m² g^? 1. X-ray fluorescence analyses revealed that schwertmannite formed in solution with initial pH in the range of 2.00–3.44 had similar elemental composition and could be expressed as Fe₈O₈(OH)_4.42(SO₄)_1.79, whereas Fe₈O₈(OH)_4.36(SO₄)_1.82 and Fe₈O₈(OH)_4.29(SO₄)_1.86 as its chemical formula when the initial pH was 1.80 and 1.60, respectively.     

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.     

Room temperature synthesis of nearly monodisperse rodlike rutile TiO2 nanocrystals

A new method was developed to synthesize uniform rodlike rutile TiO₂ nanocrystals by the hydrolysis of tetrabutyl titanate [Ti(OC₄H₉)₄] in hydrochloric acid-alcohol aqueous solutions at room temperature. The hydrolytic sol-gel reaction generated 44 nm (diameter) × 200 nm (length) sized rutile TiO₂ nanocrystals. Transmission electron microscopic images showed that the particles have a uniform shape and narrow size distribution. X-ray diffraction and electron diffraction patterns combined with high-resolution transmission electron microscopic image showed that the rodlike TiO₂ nanoparticles prepared at room temperature were crystalline rutile structure grown along the [001] direction. The morphology and photocatalytic activity of the TiO₂ nanocrystals formed at different urea concentrations were showed. The rutile TiO₂ nanocrystals formed in the absence of urea exhibited higher photocatalytic activity than the commercial photocatalyst P25 on the photocatalytic degradation of Rhodamine B.     

Preparation of core-shell structured amorphous aluminum hydroxide ultra-fine powders via a microwave-hydrothermal route

By using Al₂(SO₄)₃ aqueous solution and urea as raw materials, spherical and laminar morphology amorphous aluminum hydroxide (Al(OH)₃) ultra-fine powders were synthesized after 120 min reaction time at 150 °C via a microwave-hydrothermal route. It is interesting to note that the spherical particles could be self-encapsulated into the hollow shells which were constructed with the laminar morphology particles to form core-shell structured ultra-fine powders. The microscope analysis revealed that the spherical cores were around 1 μm and the shells were about 2-3 μm in diameter, respectively. To investigate the influencing factors and formation mechanism of the as-obtained core-shell structured amorphous aluminum hydroxide (Al(OH)₃) ultra-fine powders, samples subjected to different reaction durations were also studied.