Morphological control of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> particles via glycothermal process

Acicular magnetite (Fe₃O₄) powders were synthesized through new glycothermal dehydration by using crystalline α-FeOOH as precursor and glycols as solvent. When ethylene glycol was used as solvent, the phase was in-situ transformed from acicular α-FeOOH to α-Fe₂O₃ and finally to Fe₃O₄ at 270 °C for 6 h without morphological change. When water was added as a co-solvent in glycothermal reaction, Fe₃O₄ powders were synthesized through dissolution–recrystallization process at 230 °C for 3 h. The volume ratio of ethylene glycol to water (E/W) in the reaction has a strong effect on the morphology of the synthesized Fe₃O₄ particles. The particle shape of Fe₃O₄ particles changed from needle to sphere when the water content in E/W volume ratio increased from 0.5 to 1 mL in mixed glycothermal condition. When the water were added by more than 10 ml, the particle shape of Fe₃O₄ changed from sphere to octahedron truncated with the {100} faces and finally distinct octahedron with only {111} faces. Also, it is demonstrated that the size of Fe₃O₄ particles can be controlled from 1–2 μm to 100–200 nm by varying the reaction conditions such as the volume ratio of water to ethylene glycol and additive in glycothermal reaction.     

Solvent thermal synthesis and gas-sensing properties of Fe-doped ZnO

In this study, pure ZnO microbullets, ZnO–ZnFe₂O₄ composite, and ZnO–Fe₂O₃–ZnFe₂O₄ composite with micron structured balloons, rods, and particles were prepared by a simple solvent thermal process using methanol or ethanol as solvents. The influence of solvents on the composition and morphology of the products was studied, and their gas-sensing properties were also investigated. The morphology of ZnO microbullets synthesized in ethanol is similar to but more uniform than that of ZnO microbullets synthesized in methanol. The Fe-doped ZnO synthesized in ethanol contains many micron particles homogeneously dispersing on the surface of the microbullets, which is composed of hexagonal wurtzite ZnO and franklinite ZnFe₂O₄, while Fe-doped ZnO prepared in methanol consists of micron structured balloons, rods, and particles, which is composed of hexagonal wurtzite ZnO, hematite Fe₂O₃, and franklinite ZnFe₂O₄. Compared with pure ZnO and ZnO–ZnFe₂O₄ composite, the ZnO–Fe₂O₃–ZnFe₂O₄ composite presented high response, rapid response/recovery characteristics, good selectivity, and excellent stability to acetone at relatively low operating temperature of 190 °C. This sensor could detect acetone in wide range of 1–1000 ppm, which was expected to be a promising gas sensor for detecting acetone.     

Effect of chromium on the phase evolution and microstructure of ZnO doped with bismuth and antimony

Crystalline phase formation and microstructure of ZnO varistors with a basic composition ZnO–Bi₂O₃–Sb₂O₃ were examined. Addition of chromium oxide to this basic varistor resulted in an α-spinel (α-Zn₇Sb₂O₁₂) phase dissolving a significant amount of Cr, while the β-spinel did not. β-spinel transformed to pyrochlore during cooling, whereas α-spinel hardly transformed to pyrochlore irrespective of the cooling conditions. When Sb₂O₃ was completely replaced by Cr₂O₃, ZnCr₂O₄ was formed instead of spinel. α-spinel particles were 1–2 μm in size and intra- as well as intergranular. ZnCr₂O₄ particles, smaller than 1 μm in size, however, were present as aggregates in the bismuth-rich matrix phase at the grain boundaries. This revised version was published online in November 2006 with corrections to the Cover Date.     

Sintering ofβ-sialon with 5mol% Y2O3-ZrO2 additives

The sintering behaviour ofβ-Sialon composition powders with 5 mol% Y₂O₃-ZrO₂ additives at 1750°C for 1.5 h in nitrogen or argon atmospheres was studied.β-Sialon composition powders could be pressureless-sintered to about 93% theoretical density by the addition of 5 wt% 5 mol% Y₂O₃-ZrO₂. By HIPing the pressureless-sintered bodies the density was increased to higher than 98% theoretical density, and uniform submicrometre ZrO₂ particles were homogeneously dispersed in theβ-Sialon matrix, resulting in an increase of fracture toughness,K _1c, from 5.1 to about 5.7 MN m^−1.5. Increasing the amount of tetragonal ZrO₂ transformable to monoclinic phase in theβ-Sialon matrix increasedK _1c.     

Improved structural stability of titanium-doped β-Bi2O3 during visible-light-activated photocatalytic processes

Due to its strong absorption to visible light and intrinsic polarizability, β-Bi₂O₃ could be a promising candidate for the visible-light-activated photocatalysis. However, its structural instability during a photocatalytic process prevents it from being used practically. In this work, titanium-doped β-Bi₂O₃ was synthesized by a hydrothermal method with subsequent calcination under 400 °C. Its crystal structure, photophysical property, and structural stability were investigated by using powder X-ray diffraction, Raman, infrared and diffuse reflectance UV–vis spectroscopies. The crystal structure of the titanium-doped β-Bi₂O₃ is analogous to β-Bi₂O₃. These two oxides exhibited comparable photocatalytic activities on the photodegradation of indigo carmine, rhodamine B, and methylene blue under visible-light irradiation. However, unlike β-Bi₂O₃, the titanium-doped β-Bi₂O₃ was quite stable during these photocatalytic reactions. The improvement in structural stability was attributable to the substitution of titanium species in the host crystal lattice. The current investigation results point toward the possibility of metal ion-doped bismuth oxides as efficient visible-light-activated photocatalysts.     

Synthesis and vibrational properties of hematite (α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>) nanoparticles

α- Fe₂O₃ nanoparticles have been synthesized by gel evaporation method in air at 300°C. The average size of as synthesized α-Fe₂O₃ nanoparticle was estimated to be 30 nm and the particles were of good crystalline nature. Shape of the nanoparticles were slightly deviated from spherical which is attributed to the asymmetric growth of primary nuclei. MicroRaman and X-ray diffraction results have shown mixed phases of α-Fe₂O₃ and γ-Fe₂O₃. However, the α-Fe₂O₃ phase is more predominant than γ-Fe₂O₃ due to the incomplete nucleation of α-Fe₂O₃ particles at the size of 30 nm. The vibrating sample magnetometer measurement shows that the nanoparticles possess ferromagnetic property.     

Fabrication of high strength β-sialon by reaction sintering

The reaction sintering of β-sialon (Si₄Al₂O₂N₆) from a powder mixture of Si₃N₄, Al₂O₃ and AIN was studied to clarify factors affecting the densification. The presence of sufficient SiO vapour on the compact and excess oxide with respect toβ-sialon composition was the most important factor. High densityβ-sialon was fabricated by heating the compact at 1800° C under 1 atm. N₂. The sintering was carried out with sufficient SiO vapour pressure to prevent thermal decomposition of sintered sialon by packing the compact with a powder mixture of Si₃N₄ and SiO₂. Care was taken to minimize the amount of excess oxide in the starting composition to obtain a high density sialon with a small amount of intergranular X-phase. The maximum density of 3.04 g cm⁻³ was obtained from the compact with 2 wt % excess Al₂O₃ in the composition. The strength of the sinteredβ-sialon was 490 MN m⁻² at room temperature and 480 MN m⁻² at 1200° C. The values are the best among those so far published for sinteredβ-sialons.     

A simple approach for the synthesis of Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanocrystals

By using Co²⁺ and Co³⁺ salts, and freshly extracted ovalbumin, Co₃O₄ nanocrystals have been synthesized successfully. The pH of the solution was self-regulated for the hydrolysis of metal ions as the ovalbumin-water mixture was highly basic. Water soluble ovalbumin proteins served as a perfect matrix for entrapment of Co²⁺ and Co³⁺ ions thus forming a gel. Upon heat treatment, the dried gel precursor decomposed into nanocrystalline Co₃O₄. The crystallite size obtained by XRD line profile fitting was 45 ± 8 nm and particle size estimated from the SEM was in the range 20 nm-2 μm. EPR results show a very good fit to literature reports for nanocrystals in the size range of 8–17 nm. Even though the overall particle size is quite large and its distribution is quite wide EPR results confirm nanocrystalline nature of the particles obtained. Presented route is simple, cost effective, and environmentally friendly.     

Synthesis and properties of in situ Si3N4-reinforced BaO·Al2O3·2SiO2 ceramic matrix composites

Silicon nitride (94.5% α, 5.5% β), BaCO₃, Al₂O₃, and SiO₂ powders were mixed and pressureless sintered to produce a ceramic matrix composite consisting of 30 vol% barium aluminosilicate (BaO·Al₂O₃·2SiO₂ or BAS) matrix reinforced with in situ grown whiskers of β-Si3N4. In situ X-ray studies of the reactions indicated that BaCO₃ decomposes first to yield BaO which reacts with SiO₂ to yield a series of barium silicates which then react with Al₂O₃ between 950 and 1300°C to yield hexacelsian BAS. The sintering times were varied in order to develop a material system that combines the favourable properties of BAS with the high strength of Si₃N₄. In situ high-temperature X-ray studies after composite processing did not reveal any changes in the BAS or Si3N4 up to temperatures of 1300°C. Dilatometry studies of the sintered composite indicated a low-temperature transformation between 230 and 260°C with the temperature of transformation and volume change associated with the hexagonal to orthorhombic transformation decreasing with an increase of sintering time. Room- and high-temperature (1400°C) strengths were evaluated using four-point bend flexural tests. Composites exhibited near theoretical densities and an increase in flexural strength that was primarily dependent on the higher α- to β-Si₃N₄ transformation. This revised version was published online in November 2006 with corrections to the Cover Date.     

Thermoanalytical study of the polymorphism and melting behavior of Cu<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>

We have developed a procedure for the synthesis of phase-pure α- and β-Cu₂V₂O₇. Thermal analysis and X-ray diffraction demonstrate that the β-phase (monoclinic structure) exists at low temperatures (stability range 25–610°C), while α-Cu₂V₂O₇ (orthorhombic structure) is stable in the range 610–704°C. The α-phase observed during cooling, in particular at room temperature, is in a metastable state. The melting of the high-temperature phase γ-Cu₂V₂O₇, which forms between 704 and 716°C, has the highest rate in the range 770–785°S and is accompanied by peritectic decomposition and oxygen gas release. Subsequent cooling gives rise to four exothermic peaks, one of which (780.9°C) is attributable to the crystallization of the peritectic melt, one (620.1°C) is due to the γ → α → β phase transformations of Cu₂V₂O₇, and the other two arise from the crystallization of multicomponent low-melting-point eutectics containing α- and β-Cu₂V₂O₇, CuVO₃, and other compounds.     

Synthesis and electrophoretic deposition of magnetic nickel ferrite nanoparticles

Powders with particle size ∼5–15 nm of nickel ferrite have been synthesized chemically from aqueous precursor solutions. From the structural and magnetic properties, it is determined that the synthetic material possesses high NiFe₂O₄ phase purity and controllable particle size. The optimum calcination temperature is found to be ∼500 °C, at which the NiFe₂O₄ particles exhibit a saturation magnetization of 2800 G, and a particle size of about 10 nm. The particles are then deposited onto silicon substrates by electrophoretic deposition (EPD) process. The Ni ferrite particles are suspended in a medium of isopropyl alcohol with magnesium nitrate and lanthanum nitrate salts as charging agents. The transportation of particles to the substrate surface is assisted by applied electric field and particles adhere to the substrate surface by a glycerol based surfactant. The magnetic response of the EPD film has been investigated by vibrating sample magnetometer (VSM) measurements.     

Synthesis and photoluminescence properties of LiEu(W,Mo)<Subscript>2</Subscript>O<Subscript>8</Subscript>:Bi<Superscript>3+</Superscript> red-emitting phosphor for white-LEDs

LiEu_1−x (W_2−y Mo _y )O₈:xBi³⁺ series red-emitting phosphors were synthesized by solid state reaction. The structure, morphology, and photoluminescent properties of phosphors were studied by X-ray powder diffraction, scanning electron microscopy, and photoluminescence spectrum, respectively. X-ray powder diffraction analysis showed that the as-obtained phosphors belong to the scheelite structure. The average particle size of the investigated phosphor was about 8 μm. The excitation spectrum exhibits a charge-transfer broad band along with some sharp peaks from the typical 4f–4f transitions of Eu³⁺. Under excitation of UV, near-UV, or blue light, these phosphors showed strong red emission at 615 nm due to ⁵D₀–⁷F₂ transition of Eu³⁺. The incorporation of Mo⁶⁺ into LiEuW₂O₈:Bi³⁺ could induce red-shift of the charge-transfer broad band and a remarkable increase of photoluminescence. The highest red-emission intensity was observed with LiEu_0.80Mo₂O₈:0.20Bi³⁺. Compared with the commercial red-emitting phosphor, Y₂O₂S:Eu³⁺, the emission intensity of LiEu_0.80Mo₂O₈:0.20Bi³⁺ phosphor is much stronger than that of Y₂O₂S:Eu³⁺ and its chromaticity coordinates are closer to the standard values than that of the commercial phosphor. The optical properties of LiEu_0.80Mo₂O₈:0.20Bi³⁺ phosphor make it attractive for the application in white-light-emitting diodes (LEDs), in particular for near-UV InGaN-based white-LEDs.     

Dielectric properties of nanophase Ag2HgI4 and Ag2HgI4-Al2O3 nanocomposites

The dielectric properties of nanophase Ag₂HgI₄ and Ag₂HgI₄-Al₂O₃ nanocomposites at different frequencies have been studied over a temperature range covering the stability range of β phase of Ag₂HgI₄ and beyond the β to a phase transition temperature. έ′, tan δ and σ_a.c. of nanophase Ag₂HgI₄ and Ag₂HgI₄-Al₂O₃ nanocomposites were found to be larger than the reported values for polycrystalline pellets of Ag₂HgI₄. The dielectric properties of the nanocomposites were found to be a function of the wt.% of nano alumina. The observed changes are attributed to the grain boundary properties of nanophase materials and to the microsize space charge effects.     

Facile Synthesis of Co<Subscript>3</Subscript>O<Subscript>4</Subscript>/CoO Nanoparticles by Thermal Treatment of Ball-Milled Precursors

Co₃O₄/CoO nanoparticles have been synthesized by a simple method which is based on the ball-milling and calcination of cobalt acetate and citric acid. The samples were characterized using X-ray diffraction, transmission electron microscope, and Fourier transform infrared spectroscopy. The results show that Co₃O₄ nanoparticles with an average particle size of ∼40 nm can be obtained by calcination of ball-milled precursors at relatively low temperature (350 °C) for 3 hours. It should be noted that it is possible to control the size of Co₃O₄ particles by calcination temperature, calcination time and also by ball-milling duration using this method. Meanwhile, the pure CoO nanoparticles were obtained successfully by thermal decomposition of Co₃O₄ at 950 °C and quickly quenching to liquid nitrogen.     

Enzymatic preparation of hollow magnetite microspheres for hyperthermic treatment of cancer

Ferrimagnetic materials can be expected to be useful as thermal seeds for hyperthermic treatment of cancer, especially where the cancer is located in deep parts of body, as they can generate heat by magnetic hysteretic loss when they are placed in an alternating magnetic field. In this study, hollow magnetite (Fe₃O₄) particles were prepared using an enzymatic reaction of urease. A hollow particle was obtained by using a Pasteur pipette. The particle was 500 μm in size and was composed of Fe₃O₄. Its saturation magnetization and coercive force were 57 emu⋅g⁻¹ and 183 Oe, respectively. Its heat generation under an alternating magnetic field of 300 Oe at 100 kHz was estimated to be 45 W⋅g⁻¹. Microspheres 30 μm in diameter were also successfully obtained by using a spray gun.     

Phase diagram of the system Ca-Ti-O at 1200 K

Phase relations in the system Ca-Ti-O have been established by equilibration of several samples at 1200 K for prolonged periods and identification of phases in quenched samples by optical and scanning electron microscopy, XRD and EDS. Samples representing 20 compositions in the ternary system were analyzed. There was negligible solid solubility of Ca in the phases along the binary Ti-O, and of Ti in CaO. Four ternary oxides were identified: CaTiO₃, Ca₄Ti₃O₁₀ and Ca₃Ti₂O₇ containing tetravalent titanium, and CaTi₂O₄ containing trivalent titanium. Tie-lines link calcium titanite (CaTi₂O₄) with the three calcium titanates (CaTiO₃, Ca₄Ti₃O₁₀ and Ca₃Ti₂O₇), CaO, oxygen excess TiO_1+δ and stoichiometric TiO. Tie-lines connect CaTiO₃ with TiO_2−x , Magneli phases Ti _n O_2n−1 (28 ≥ n ≥ 4), Ti₃O₅, Ti₂O₃ and TiO_1+δ . CaO was found to coexist with TiO, and Ti-O solid solutions α and β. The phase diagram is useful for understanding the mechanisms and kinetics of direct calciothermic reduction of TiO₂ to metal and electrochemical reduction of TiO₂ using graphite anode and molten CaCl₂ electrolyte.     

Crystallization behaviour of β-spodumene in the calcination of Li2O-Al2O3-SiO2-ZrO2 gels

This work was carried out in order to prepare precursor powders with a spodumene composition (Li₂O·Al₂O₃·4SiO₂, LAS) and to investigate their crystallization behaviours during calcination. A fine β-spodumene type amorphous powder was obtained through sol-gel techniques using LiOCH₃, Al(OC₂H₅)₃, Si(OC₂H₅)₄ and Zr(OC₂H₅)₄ as the starting metal alkoxides. The process included well controlled hydrolysis polycondensation of the raw alkoxides. Differential thermal analysis (DTA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron diffraction (ED) analyses were utilized to study the crystallization behaviour of the gels. The activation energy of β-spodumene crystallization was 192 kJ mol⁻¹ for LAS gels with 4 wt% ZrO₂, being much smaller than those of LAS gel without ZrO₂, 382 kJmol⁻¹. For calcination at 800–1200°C, the crystallized phases comprised a major phase of β-spodumene and a minor phase of zirconia (ZrO₂).     

Formation of plate-like lanthanum-β-Aluminate crystal in Ce-TZP matrix

The reaction route and morphology of lanthanum--aluminate (LBA) crystals formed in Ce-TZP matrix were studied by examining the crystal phase changes and the microstructures in relation to the heat-treatment time, heat-treatment temperatures and the particle size of raw Al₂O₃ powders. In the Ce-TZP matrix, the LBA crystal was formed by the reaction between La₂O₃ and Al₂O₃ through the LaAlO₃ phase as the intermediate. La₂Zr₂O₇ forms at 800 °C and remains in the temperature range 800–1500 °C, and LaAlO₃ forms between 1200 and 1400 °C. The LaAlO₃ reacts with Al₂O₃ to form LBA above 1500 °C. The diffusion of La³⁺ through the La₂Zr₂O₇ phase was faster than that of Al³⁺. The morphology of LBA crystals was dependent on the particle size of the starting raw Al₂O₃ particle. When submicrometre size Al₂O₃ (0.4m) particles were used as the starting particles, anisotropic, plate-like LBA crystals, about 10m long, were formed during heat treatments. On the other hand, Al₂O₃ of larger grain sizes (3.6, 10.3m) yield conglomerates of LBA crystals. The size of the conglomerates is similar to that of the raw Al₂O₃ particle. The dependence of the morphology of LBA on the particle size of Al₂O₃ can be attributable to the sintering process of the Ce-TZP matrix, leading to the control of the mechanical properties of Ce-TZP ceramics with LBA crystals.     

Hydrothermal synthesis of a nanostructured TiO<Subscript>2</Subscript>-based material in the presence of chitosan

Quasi-one-dimensional TiO₂-based nanostructures have been produced through hydrothermal treatment-without additives and in the presence of chitosan—of anatase nanopowder synthesized by an electrochemical sol-gel process. The morphology, phase composition, and structure of the hydrothermal synthesis products were studied by various physicochemical characterization techniques, including high-resolution electron microscopy, X-ray diffraction, and IR spectroscopy. The results demonstrate that the forming one-dimensional structures are isostructural with β-titanic acid, H₂Ti₃O₇. Heat treatment at t ≥ 500°C yields a mixture of sodium polytitanates, Na _y Ti _x O_{2x + 1}, with y = 0.5–2 and x = 2–5. The surface morphology and shape of the nanostructures persist up to 700°C. The key features of the formation of quasi-one-dimensional TiO₂-based structures in the presence of chitosan have been identified.     

Theoretical study of the insulating oxides and nitrides: SiO2, GeO2, Al2O3, Si3N4, and Ge3N4

An extensive theoretical study is performed for wide bandgap crystalline oxides and nitrides, namely, SiO₂, GeO₂, Al₂O₃, Si₃N₄, and Ge₃N₄. Their important polymorphs are considered which are for SiO₂: α-quartz, α- and β-cristobalite and stishovite, for GeO₂: α-quartz, and rutile, for Al₂O₃: α-phase, for Si₃N₄ and Ge₃N₄: α- and β-phases. This work constitutes a comprehensive account of both electronic structure and the elastic properties of these important insulating oxides and nitrides obtained with high accuracy based on density functional theory within the local density approximation. Two different norm-conserving ab initio pseudopotentials have been tested which agree in all respects with the only exception arising for the elastic properties of rutile GeO₂. The agreement with experimental values, when available, are seen to be highly satisfactory. The uniformity and the well convergence of this approach enables an unbiased assessment of important physical parameters within each material and among different insulating oxide and nitrides. The computed static electric susceptibilities are observed to display a strong correlation with their mass densities. There is a marked discrepancy between the considered oxides and nitrides with the latter having sudden increase of density of states away from the respective band edges. This is expected to give rise to excessive carrier scattering which can practically preclude bulk impact ionization process in Si₃N₄ and Ge₃N₄.