Ultra-sharp α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoflakes: growth mechanism and field-emission

We report the synthesis of single-crystalline α-Fe₂O₃ nanoflakes from a simple Fe–air reaction within the temperatures range of 260–400 °C. The nanoflakes synthesized at the lowest temperature (260 °C) in this work show an ultra-sharp morphology: 5–10 nm in thickness, 1–2 μm in length, 20 nm in base-width and around 5 nm at the tips; successfully demonstrate the promising electron field emission properties of a large-scaled α-Fe₂O₃ nanostructure film and exhibit the potential applications as future field-emission (FE) electron sources and displays (FEDs). The formation and growth of α-Fe₂O₃ nanostructures were discussed based on the surface diffusion mechanism. PACS 79.60.Jv; 79.70.+q; 77.84.Bw     

Preparation and characteristics of?a?BaO–Al2O3–B2O3–SiO2–La2O3 glass ceramic via spray pyrolysis

The characteristics of a BaO–Al₂O₃–B₂O₃–SiO₂–La₂O₃ glass ceramic prepared by spray pyrolysis were studied. Glass powders with spherical shape and amorphous phase were prepared by complete melting at a preparation temperature of 1 500°C. The mean size and geometric standard deviation of the powders prepared at the temperature of 1 500°C were 0.6 μm and 1.3. The glass powders had similar composition to that of the spray solution. The glass transition temperature (T _g) of the glass powders was 600.3°C. Two crystallization exothermic peaks were observed at 769.3 and 837.8°C. Densification of the specimen started at a sintering temperature of 600°C, in which Ba₄La₆O(SiO₄)₆ as main crystal structure was observed. Complete densification of the specimen occurred at a sintering temperature of 800°C. The specimens sintered at temperatures above 800°C had main crystal structure of BaAl₂Si₂O₈.     

The synthesis of hafnium nanomaterials by alkali metal reduction of hafnium tetrachloride

Hafnium tetrachloride was reduced in organic solvents with lithium powder, sodium–potassium alloy, or lithium hydride/Et₃B. All reductions required sonochemical agitation to proceed at an appreciable rate, with the notable exception of the reaction of HfCl₄ with Li in diethyl ether. Activation of C–H bonds occurred in all reactions, which resulted in carbon-containing products. HfCl₄ was reduced on a 50-g scale with LiH/Et₃B, and a 10-g scale with Li powder in pentane. All the solid products from the reductions were converted to nanomaterials by annealing under vacuum from 500 to 1,000 °C, which also resulted in the sublimation of the alkali metal salts. The nanomaterials contain a mixture of products with the α-Hf (hexagonal) structure (crystallite size 8–250 nm) and the HfC (FCC) structure (crystallite size 3–80 nm), with the amount of hafnium in the bulk annealed product varying from 88 to 99 wt%. When toluene, pentane, or triethylamine solvents were used, the presence of amorphous graphitic or carbonaceous material was also detected by solid state ¹³C NMR. Thermally annealed products were additionally characterized by electron microscopy and thermal analysis under Ar/O₂, and have BET surface areas ranging from 2.7 to 155 m²/g.     

The oxidation of dodecane on a vanadium-molybdenum catalyst

The catalytic oxidation of dodecane by air oxygen on a mixed vanadium-molybdenum oxide (V₂O₅ · MoO₃, 40 mol % MoO₃) was studied over the temperature range 300–350°C. The reaction at 300–330°C occurred on oxygen vacancies with the rupture of C-H bonds and formation of α-acid. Oxidation above 350°C occurred with the splitting of the C-C bond and formation of two and more acids. Singlet oxygen ¹O₂ generated in the oxidation of oxide catalyst lattice oxygen participated in the reaction. A possible mechanism of the process was considered.     

A newly developed Fe-doped calcium sulfide nanoparticles with magnetic property for cancer hyperthermia

In this study, a magnetic iron-doped calcium sulfide (Fe–CaS) nanoparticle was newly developed and studied for the purpose of hyperthermia due to its promising magnetic property, adequate biodegradation rate, and relatively good biocompatibility. Fe–CaS nanoparticles were synthesized by a wet chemical co-precipitation process with heat treatment in a N₂ atmosphere, and were subsequently cooled in N₂ and exposed to air at a low temperature. The crystal structure of the Fe–CaS nanoparticles was similar to that of the CaS, which was identified by an X-ray diffractometer (XRD). The particle size was less than 40 nm based on a Debye–Scherrer equation and transmission electron microscope (TEM) examination. Magnetic properties obtained from the SQUID magnetometer demonstrated that the synthesized CaS was a diamagnetic property. Once the Fe ions were doped, the synthesized Fe–CaS converted into paramagnetism which showed no hysteresis loop. Having been heated above 600 °C in N₂, the Fe–CaS showed a promising magnetic property to produce enough energy to increase the temperature for hyperthermia. 10 mg/ml of the Fe–CaS was able to generate heat to elevate the media temperature over 42.5 °C within 6 min. The area of the hysteresis loop increased with the increasing of the treated temperature, especially at 800 °C for 1 h. This is because more Fe ions replaced Ca ions in the lattice at the higher heat treatment temperature. The heat production was also increasing with the increasing of heat treatment temperature, which resulted in an adequate specific absorption ratio (SAR) value, which was found to be 45.47 W/g at 37 °C under an alternative magnetic field of f = 750 KHz, H = 10 Oe. The in vitro biocompatibility test of the synthesized Fe–CaS nanoparticles examined by the LDH assay showed no cytotoxicity to 3T3 fibroblast. The result of in vitro cell hyperthermia shows that under magnetic field the Fe–CaS nanoparticles were able to generate heat and kill the CT-26 cancer cells significantly. We believe that the developed Fe–CaS nanoparticles have great potential as thermo-seeds for cancer hyperthermia in the near future.     

Phase segregation in the Fe90Zr10 amorphous alloy under heating

A study of the changes in the structure of melt-quenched Fe₉₀Zr₁₀ amorphous alloys by x-ray diffraction, Auger spectroscopy, and transmission electron microscopy is reported. The samples were subjected to isochronous (for 1 h) and isothermal anneals at 100–650 °C. It is shown that an amorphous alloy annealed for one hour at 300–500 °C crystallizes with formation of a supersaturated solid solution of Zr in α Fe and the intermetallic compound Fe₃Zr. Isothermal anneal at 100 °C for up to 7000 h produces nanocrystallites 110–30 nm in size, with fuzzy interfaces between the grains. An alloy subjected to such an anneal contains two solid solutions of Zr in Fe, having a cubic and a weakly tetragonal lattice. Crystallization taking place during low-temperature anneals is preceded by phase segregation of the alloy within the amorphous state. The lattice periods of the solid solutions have been determined. The possibility of the alloy crystallizing by spinodal decomposition during prolonged annealing is discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 1769–1772 (October 1998)     

Experimental evidences of clusters with different short range order in amorphous alloys

⁵⁶Fe,⁵⁷Fe,¹⁰B and¹¹B isotopes were used for binary alloys. The signals of B (40,5 MHz) and Fe (43 MHz) from α-Fe in binary Fe−B crystalline and amorphous alloys were found besides the signals of these nuclei in t-and o-phase or clusters like these phases. The NMR on (51)V impurity nuclei in Fe−B alloys was used as well. Only amorphous Fe-(^<15 at.%B) alloys had the clusters with o-, t-Fe₃B and α-Fe short range order.     

Thermal stabilization of tin- and cobalt-doped manganese dioxide

We study the thermal stability, local structure, and electrical properties of the α-MnO₂ phase doped with Sn and Co. It is found that doping prevents the transformation from α-MnO₂ to α-Mn₂O₃ that occurred in the temperature range of 500–600 °C. Samples have been synthesized in an acidic medium using the reduction of potassium permanganate by fumaric acid. X-ray diffraction patterns (XRD) of pure and doped α-MnO₂ prepared at 450 °C do not show new peaks related to dopant species. Thermogravimetric analysis (TGA) of the Sn and Co doped MnO₂ reveals that transformation from MnO₂ to α-Mn₂O₃ starts above 700 °C. The increase in the thermal stability is attributed to the presence of Sn or Co ions incorporated inside the large 2 × 2 tunnels as revealed by Fourier transform infrared (FTIR) spectra measurements. An increase in the electrical conductivity with the presence of Sn or Co ions is observed. Electrochemical features of the doped MnO₂ samples in alkaline cells are reported and compared with that of the pristine α-MnO₂ phase.     

The production of nickel–alumina composite coating via electroplating

A series of electroplating works have been conducted to investigate the best condition for the coelectrodeposition of nickel–alumina (Ni/α–Al₂O₃) composite coating. Co-electrodeposition was done onto mild steel as cathode at ambient temperature (27°C) with current density of 30 mA/cm² under α-Al₂O₃ concentration of 2 g/l and various agitation speeds of 50, 100, 150, 200, and 250 rpms. The cross-section of the composite coatings portrayed α-Al₂O₃ particles was co-deposited. Under field emission scanning electron microscopy analysis, the coating shows a coarse surface morphology, while cross-sectional microstructures shows a compact embedding of α-Al₂O₃ particle in the Ni matrix. Elemental analysis by EDX detected the presence of Ni and α-Al₂O₃. Vickers microhardness testing shows that the coating hardness increases almost 60% at the highest agitation speed, i.e., 250 rpm.     

Size-dependent luminescence in Y2Si2O7 nanoparticles doped with Ce3+ ions

Powders and thin films of nanocrystalline yttrium disilicate (Y₂Si₂O₇) doped with Ce³⁺ have been prepared by a sol–gel method. Structure and morphology of the synthesised samples have been determined and spectroscopic properties compared. The triclinic α-Y₂Si₂O₇ form (space group P 1-) for the powders annealed between 1000°C and 1200°C has been found. A total conversion into a thortveitite-type monoclinic β-Y₂Si₂O₇ polymorph after annealing of powder samples at 1400°C (space group C2/m) has been observed. In the case of films even at 1300°C the basically pure triclinic α-Y₂Si₂O₇ phase was observed with luminescent spectroscopy. The influence of grain size, controlled by thermal treatment, on the structure and luminescence properties of the fabricated materials are presented and discussed.     

Investigation of the state of the electrochemically generated adsorbed O species on Au films interfaced with Y2O3-doped-ZrO2

Adsorbed O species on Au interfaced with Y₂O₃-doped-ZrO₂ are generated by electrochemical O²⁻ supply. It was found that two oxygen chemisorbed states are formed, which desorb at 420 °C (state α) and 550 °C (state β) with activation energies of desorption ranging between 115–145 kJ/mol and 235–270 kJ/mol, respectively. The strong interaction of the β-state O species with the Au surface causes an over 600 mV increase in Au surface potential and work function while the α-state O species is formed at even more positive catalyst-electrode potential. State α is attributed to normally adsorbed atomic O while the more ionic state β is only created electro-chemically and is mainly responsible for the work function increase of the Au catalyst-electrode surface. Their desorption activation energies of both states decrease linearly with increasing catalyst-electrode potential with slopes of the order of four. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Synthesis and characterization of hollow ��-Fe2O3 sub-micron spheres prepared by sol�Cgel

In this work we report the preparation of magnetic hematite hollow sub-micron spheres (??-Fe₂O₃) by colloidal suspensions of ferric nitrate nine-hydrate (Fe(NO₃)₃·9H₂O) particles in citric acid solution by following the sol?Cgel method. After the gel formation, the samples were annealed at different temperatures in an oxidizing atmosphere. Annealing at 180°C resulted in an amorphous phase, without iron oxide formation. Annealing at 250°C resulted in coexisting phases of hematite, maghemite and magnetite, whereas at 400°C, only hematite and maghemite were found. Pure hematite hollow sub-micron spheres with porous shells were formed after annealing at 600°C. The characterization was performed by X-ray diffraction (XRD), Mössbauer spectroscopy (MS) and scanning electron microscopy (SEM).     

Characterization and microstructural studies of LiFe5O8 synthesized by the self propagating high temperature combustion method

LiFe₅O₈ was synthesized by the self-propagating high temperature combustion synthesis using lithium acetate and iron acetate as starting materials and glycine as reductant. The reaction was rapid when the ignition temperature was reached and the product was a light and porous mass of solid material. Analysis using X-Ray powder diffraction and SEM revealed the amorphous nature of the LiFe₅O₈ precursor material. From particle size analysis, the precursor material was much less than 100 nm, but increased upon annealing at 650 °C and 900 °C. Paper presented at the International Conference on Functional Materials and Devices 2005, Kuala Lumpur, Malaysia, June 6 – 8, 2005.     

Study of ammonia decomposition in a proton conducting solid electrolyte cell

The reaction of NH₃ decomposition was studied at 500–700 °C and atmospheric total pressure using a single-chamber cell-reactor. The proton conductor was a strontia-ceria-ytterbia (SCY) perovskite of the form: SrCe_0.95Yb_0.05O_3−α. Silver films served as cathodic and anodic electrodes. The effects of imposed current, temperature and inlet gas composition on the reaction rate were examined. Paper presented at the 8th EuroConference on Ionics, Carvoeire, Algarve, Portugal, Sept. 16–22, 2001.     

Nanocrystalline CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> powder by PVA sol–gel route: synthesis,characterization and its giant dielectric constant

Nanocrystalline CaCu₃Ti₄O₁₂ powders were synthesized by a simple PVA sol–gel route and calcined at 700 and 800°C in air for 8 h. The diameter of the powders ranges from 40–100 nm. The calcined CaCu₃Ti₄O₁₂ powders were characterized by TG-DTA, XRD, FTIR, SEM, and TEM. Sintering of the powders was conducted in air at 1100°C for 16 h. The XRD results indicated that all sintered samples had a typical perovskite CaCu₃Ti₄O₁₂ structure although the sintered samples contained some amount of CaTiO₃. SEM of the sintered CaCu₃Ti₄O₁₂ ceramics showed the average grain sizes of 13–15 μm. The samples exhibit a giant dielectric constant, ε′∼10⁵ at 150 to 200°C with weak temperature dependence below 1 kHz in the sample sintered using the powders calcined at 700°C. The Maxwell–Wagner polarization mechanism is used to explain the high permittivity in these ceramics. It is also found that all sintered samples have the same activation energy of grains, which is ∼0.122 eV.     

Effect of contacts between γ-Fe2O3 nanoparticles on their phase-transition temperature

The effect of mechanical contacts between γ-Fe₂O₃ particles on the temperature of the γ-α structural transition in them is established by magnetic studies and differential thermal analysis (DTA). The sample in which γ-Fe₂O₃ particles had no mechanical contacts with one another remained in the ferromagnetic state up to T _C = 630°C and had two exothermal DTA peaks. The first peak almost coincided with the Curie temperature, while the second peak attributed to the γ → α structural transition corresponded to 760°C. The magnetic transition for particles with a larger number of contacts was shadowed by the γ → α structural transition with a temperature lowered to 550°C.     

Mössbauer study of Fe-B alloys containing Nd additives

The influence of a few percent neodynium additives to Fe-B alloy on the hyperfine parameters was investigated. It was found that Nd decreases the hyperfine field of amorphous Fe-B alloys. The samples annealed at temperatures between 600°C and 700°C contain α-Fe and Fe₃B phases. The average hyperfine field of Fe₃B phase increases with increasing Nd content and decreases with increasing annealing temperature. The average isomer shift of Fe₃B phase decreases with increasing Nd content. The experimental data show that the change of hyperfine parameters of the Fe₃B phase in the studied alloys is due to Nd atom.     

Microwave-assisted synthesis and magnetic property of magnetite and hematite nanoparticles

Nanoparticles of magnetite (Fe₃O₄) and hematite (α-Fe₂O₃) have been prepared by a simple microwave heating method using FeCl₃, polyethylene glycol and N₂H₄·H₂O. The amount of N₂H₄·H₂O has an effect on the final phase of Fe₃O₄. The morphology of α-Fe₂O₃ was affected by the heating method. Crystalline α-Fe₂O₃ agglomerates were formed immediately at room temperature and most of these nanoparticles within agglomerates show the same orientation along [110] direction. After microwave heating, ellipsoidal α-Fe₂O₃ nanoparticles were formed following an oriented attachment mechanism. Both Fe₃O₄ and α-Fe₂O₃ nanoparticles exhibit a small hysteresis loop at room temperature.     

Network structure and thermal property of a novel high temperature seal glass

In this study, novel glasses based on SrO–La₂O₃–Al₂O₃–B₂O₃–SiO₂ system are investigated for solid oxide fuel and electrolyzer cells. The network structure evolution of the glasses with increasing B₂O₃:SiO₂ ratio was studied using Raman spectroscopy. The thermal properties of the glasses, including glass transition temperature T _g and glass softening temperature T _d , were studied using dilatometry. The thermal stability of the glasses was investigated using X-ray diffraction. The study shows that as the B₂O₃:SiO₂ ratio increases, the SrO–La₂O₃–Al₂O₃–B₂O₃–SiO₂ glass micro-heterogeneity and the amount of non-bridging oxygen atoms increase. Correspondingly, the T _g of the SrO–La₂O₃–Al₂O₃–B₂O₃–SiO₂ glasses changes from 635 to 775°C, and the T _d changes from 670 to 815°C. Glass thermal stability decreases with B₂O₃:SiO₂ ratio increase. The glass without B₂O₃ is thermally stable after being kept at 850°C for 200 hrs.     

Sonochemical assisted synthesis of SrFe12O19 nanoparticles

We present the synthesis of M-type strontium hexaferrite by sonochemistry and annealing. The effects of the sonication time and thermal energy on the crystal structure and magnetic properties of the obtained powders are presented. Strontium hexagonal ferrite (SrFe₁₂O₁₉) was successfully prepared by the ultrasonic cavitation (sonochemistry) of a complexed polyol solution of metallic acetates and diethylene glycol. The obtained materials were subsequently annealed at temperatures from 300 to 900 °C. X-ray diffraction analysis shows that the sonochemical process yields an amorphous phase containing Fe³⁺, Fe²⁺ and Sr²⁺ ions. This amorphous phase transforms into an intermediate phase of maghemite (γ-Fe₂O₃) at 300 °C. At 500 °C, the intermediate species is converted to hematite (α-Fe₂O₃) by a topotactic transition. The final product of strontium hexaferrite (SrFe₁₂O₁₉) is generated at 800 °C. The obtained strontium hexaferrite shows a magnetization of 62.3 emu/g, which is consistent with pure hexaferrite obtained by other methods, and a coercivity of 6.25 kOe, which is higher than expected for this hexaferrite. The powder morphology is composed of aggregates of rounded particles with an average particle size of 60 nm.