Synthesis of rare earth hydroxide nanorods by room temperature reaction of oxide precursors with water

In this study, solid state chemical reaction of anhydrous chloride precursors with Na₂CO₃ has been used to manufacture composite powders of nanocrystalline rare earth oxide grains embedded within a matrix of NaCl. Subsequent washing of these powders with deionised water resulted in removal of the NaCl matrix and hydration of the oxide nanoparticles. In the case of Nd₂O₃ and Sm₂O₃, the hydration process yielded hydroxide nanorods. In contrast, washing of the Gd₂O₃ yielded a powder consisting of irregularly shaped nanoparticles of Gd₂O₃ and GdOOH. Analysis using high resolution transmission electron microscopy revealed that the Nd₂O₃ and Sm₂O₃ nanorods were not single crystals but were composed of crystalline subunits with a common á 0001 ñ \left\langle {0001} \right\rangle growth direction. The results obtained in this study demonstrate that rare earth hydroxide nanorods can be synthesised from oxide precursors without the need for hydrothermal processing at elevated temperature and pressure.     

Synthesis of CaCO3 Nanoparticles by Mechanochemical Processing

The synthesis of calcite (CaCO₃) nanoparticles by mechanochemical reaction and subsequent heat treatment was investigated. A solid-state displacement reaction CaCl₂ + Na₂CO₃ CaCO₃+2NaCl was induced during mechanical milling of a CaCl₂+ Na₂CO₃ powder mixture. Heat treatment of the as-milled powder at 350°C completed the reaction, forming crystalline CaCO₃ nanoparticles separated from each other in a dry-salt matrix. A simple washing process to remove the matrix yielded calcite single phase ultrafine powder. The mean particle size was controlled by changing the volume fraction of CaCO₃ in the matrix. 20% volume fraction yielded nanoparticles of ~ 140 nm in size, whereas 10% volume fraction led to ~ 80 nm size nanoparticles.     

Microstructure characterization of mechanosynthesized nanocrystalline NiFe2O4 by Rietveld's analysis

Nanocrystalline nickel ferrite (NiFe₂O₄) is synthesized at room temperature by high-energy ball milling the stoichiometric mixture of (1:1 mol%) of NiO and α-Fe₂O₃ powders. The structural and microstructural evolution of NiFe₂O₄ caused by milling is investigated by X-ray powder diffraction. The relative phase abundance, particle size, r.m.s. strain, lattice parameter changes of different phases have been estimated employing Rietveld structure refinement analysis of X-ray powder diffraction data. Particle size and content (wt%) of both NiO and α-Fe₂O₃ phases reduce rapidly with increasing milling time and a significant amount of nanocrystalline NiFe₂O₄ is formed within 1 h of ball milling. Particle sizes of all the phases reduce to ∼10 nm within 5 h of milling and remain almost unchanged with increasing milling time up to 20 h. Lattice parameter of cubic NiO decreases linearly with increasing milling time, following the Vegard's law of solid-solution alloy. A continuous decrease in lattice parameter of cubic NiFe₂O₄ phase clearly suggests that smaller Ni atoms have occupied some of the vacant oxygen sites of ferrite lattice. Cation distribution both in octahedral and tetrahedral sites changes continuously with milling time and the normal spinel lattice formed at early stage of milling, transforms to inverse spinel lattice in the course of milling. High-resolution transmission electron microscope (HRTEM) micrographs of 11 h milled sample corroborates the findings of X-ray profile analysis.     

Effect of Particle Size on the Photocatalytic Activity of Nanoparticulate Zinc Oxide

In this study, a three-stage process consisting of mechanical milling, heat treatment, and washing has been used to manufacture nanoparticulate ZnO powders with a controlled particle size and minimal agglomeration. By varying the temperature of the post-milling heat treatment, it was possible to control the average particle size over the range of 28–57 nm. The photocatalytic activity of these powders was characterized by measuring the hydroxyl radical concentration as a function of irradiation time using the spin-trapping technique with electron paramagnetic resonance spectroscopy. It was found that there exists an optimum particle size of approximately 33 nm for which the photocatalytic activity is maximized. The existence of this optimal particle size is attributable to an increase in the charge carrier recombination rate, which counteracts the increased activity arising from the higher specific surface area for a sufficiently small particle size.     

Fully dense nano-composite energetic powders prepared by arrested reactive milling

Highly reactive metastable nano-scale composites of aluminum and metal oxides have been produced by arrested reactive milling (ARM), and their combustion performance has been preliminarily evaluated. Aluminum powder has been milled with powders of MoO₃ and Fe₂O₃. The prepared composites are powders with particle sizes in the 1–100 μm range. Each individual particle comprises a fully dense, nano-scale mixture of the chemical reagents. These composites belong to a novel class of energetic materials characterized by an intimate mixing of reactive components on nanometer to atomic scale. Reactive components can be metal/metal oxide pairs or combinations of other materials capable of highly exothermic reactions such as B–Ti or B–Zr. High-energy milling of these components leads to mechanical initiation of the reaction. Highly reactive composites are obtained by arresting this process immediately before the initiation would occur if milling were allowed to proceed. An experimental parametric study of reactive milling in the Al–MoO₃ and Al–Fe₂O₃ systems was conducted to establish at which milling times the reaction is spontaneously initiated under various conditions. Samples of nano-composite powders were synthesized by arresting the milling process, and characterized using electron microscopy, X-ray diffraction, and particle size analysis. Ignition temperatures of the materials were determined at heating rates in the range of 300–3000 K/s using an electrically heated filament. Activation energies of ignition were determined to be 152 ± 19 and 170 ± 25 kJ/mol for the Al-MoO₃ and Al-Fe₂O₃ nano-composites, respectively. The activation energy obtained for the Al-Fe₂O₃ nano-composite is consistent with a previously reported value for the Al-Fe₂O₃ thermite reaction. Combustion tests were conducted in a constant volume pressure vessel in argon for both Al-Fe₂O₃ and Al-MoO₃ and compared to respective blends of initial powders and to partially milled powders. The nano-composites showed higher respective reaction rates. Linear burning rates measured in an open channel of 2.5 × 2.5 mm cross-section were also higher for the ARM-prepared powders compared to partially milled materials.     

A comparative evaluation of the photocatalytic and optical properties of nanoparticulate ZnO synthesised by mechanochemical processing

In this study, mechanochemical processing has been used to manufacture a nanoparticulate powder of ZnO with a controlled particle size and minimal hard agglomeration. The suitability of this ZnO powder for use as either a photocatalyst or an optically transparent UV-filter was evaluated by comparing its optical and photocatalytic properties with those of three commercially available powders that were synthesised by chemical precipitation and flame pyrolysis. The ZnO powder synthesised by mechanochemical processing was found to exhibit high optical transparency and low photocatalytic activity per unit of surface area, which indicates that it is suitable for use in optically transparent UV-filters.

     

Study of nanocrystalline and amorphous powders prepared by mechanical alloying

The process of mechanical alloying consists of intimate mixing and mechanical working of elemental powders in a high-energy ball mill. It has been well established that this process is able to produce nanocrystalline and amorphous material. In this study, the structural effects of mechanical alloying of pure Fe, Fe₅₀W₅₀ and Fe₅₀Mo₅₀ powders were investigated by X-ray diffraction and Mössbauer spectroscopy. For all cases, nanocrystalline and/or amorphous fractions were found after milling. The resulting particle size was determined by X-ray diffraction. Pure Fe does not amorphize even after prolonged milling times. For the nanocrystalline powder, significant changes in the linewidth and the hyperfine field are found. Powder mixtures of Fe₅₀Mo₅₀ and Fe₅₀W₅₀ are completely amorphous after milling times of 10 h, as seen by Mössbauer spectroscopy, but nanocrystalline fractions of the non-iron part are still found in X-ray diffraction. Also in the amorphous state, further changes in the hyperfine parameters are found with increasing milling time.     

Microstructure of nanocrystalline nonstoichiometric vanadium carbide VC0.875

A nanocrystalline powder of nonstoichiometric vanadium carbide VC_0.875 has been prepared by the high-energy ball milling method. The crystal structure, microstructure, morphology, and size distribution of particles of the initial and milled powders have been investigated using X-ray diffraction, laser diffraction, and scanning electron microscopy. For vanadium carbide, the model calculation of the particle size of a VC_0.875 nanopowder as a function of the milling duration has been performed for the first time. A comparison of the experimental and theoretical results has demonstrated that a nanopowder with an average particle size of 40–80 nm can be obtained by a 10-h high-energy ball milling of the initial vanadium carbide powder with an average particle size of ～6 μm.     

X-ray characterization and phase transformation kinetics of ball-mill prepared nanocrystalline Mg–Zn-ferrite at elevated temperatures

Nanocrystalline Mg–Zn-ferrite is prepared by ball milling the stoichiometric powder mixture of MgO, ZnO and α-Fe₂O₃. A non-stoichiometric ferrite phase is noticed to form after 3 h of milling when particles of starting materials became nano-sized. After 25 h of milling, stoichiometric ferrite phase is formed with 9 nm particle size. Post annealing study of ball-milled sample reveals that the nanocrystalline ferrite phase is stable up to 873 K and then starts to decompose into individual starting phases. However, heat treatment of unmilled stoichiometric powder mixture even at 1473 K for 1 h duration does not result in formation of stoichiometric Mg–Zn-ferrite phase.     

Synthesis and characterization of indium oxide-hematite magnetic ceramic solid solution

Indium oxide-doped hematite xIn₂O_3*(1-x)??-Fe₂O₃ (molar concentration x = 0.1?C0.7) solid solutions were synthesized using mechanochemical activation by ball milling. XRD patterns yield the dependence of lattice parameters and grain size as function of milling time. After 12 h of milling, the completion of In^3?+? substitution of Fe^3?+? in hematite lattice occurs for x = 0.1. For x = 0.3, 0.5 and 0.7, the substitutions between In^3?+? and Fe^3?+? into hematite and respectively, In₂O₃ lattices occur simultaneously. The lattice parameters of ??-Fe₂O₃ (a and c) and In₂O₃ (a) vary with milling time. For x = 0.1, Mössbauer spectra were fitted with one, two, or three sextets versus milling time, corresponding to gradual substitution of In^3?+? for Fe^3?+? in hematite lattice. For x = 0.3, Mössbauer spectra after milling were fitted with three sextets and two quadrupole-split doublets, representing In^3?+? substitution of Fe^3?+? in hematite lattice and Fe^3?+? substitution of In^3?+? in two different sites of In₂O₃ lattice. For x = 0.5 and 0.7, Mössbauer spectra fitting required two sextets and one quadrupole-split doublet, representing coexistence of In^3?+? substitution of Fe^3?+? in hematite lattice and Fe^3?+? substitution of In^3?+? in indium oxide lattice. The recoilless fraction studied versus milling time for each molar concentration exhibited low values, consistent with the occurrence of nanoparticles in the system. SEM/EDS measurements revealed that the mechanochemical activation by ball milling produced xIn₂O_3*(1-x)??-Fe₂O₃ solid solution system with a wide range of particle size distribution, from nanometer to micrometer, but with a uniform distribution of Fe, In, and O elements.     

Optical and photocatalytic properties of nanocrystalline TiO2 synthesised by solid-state chemical reaction

Nanoparticulate TiO₂ is of interest for a variety of technological applications, including optically transparent UV-filters and photocatalysts for the destruction of chemical waste. The successful use of nanoparticulate TiO₂ in such applications requires an understanding of how the synthesis conditions effect the optical and photocatalytic properties. In this study, we have investigated the effect of heat treatment temperature on the properties of nanoparticulate TiO₂ powders that were synthesised by solid-state chemical reaction of anhydrous TiOSO₄ with Na₂CO₃. It was found that the photocatalytic activity increased with the heat treatment temperature up to a maximum at 600 °C and thereafter declined. In contrast, the optical transparency decreased monotonically with the heat treatment temperature. These results indicate that solid-state chemical reaction can be used to prepare powders of nanoparticulate TiO₂ with properties that are optimised for use as either optically transparent UV-filters or photocatalysts.     

Preparation and dielectric properties of the lead-free BaFe<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>O<Subscript>3</Subscript> ceramics obtained from mechanically triggered powder

In the paper the influence of mechanical activation of the powder on the final dielectric properties lead-free Ba(Fe_1/2Nb_1/2)O₃ (BFN) ceramic was examined. The BFN ceramics were obtained by 3-steps route. Firstly, the substrates were pre-homogenized in a planetary ball mill. Than, the powder was activated in vibratory mill (the shaker type SPEX 8000 Mixer Mill) for different duration between 25 h and 100 h. The influence of the milling time on the BFN powder was monitored by X-ray diffraction. The diffraction data confirmed that the milling process of the starting components is accompanied by partial synthesis of the BFN materials. The longer of the high-energy milling duration the powders results in increasing the amount of amorphous/nanocrystalline content. The mechanically activated materials were sintered in order to obtain the ceramic samples. During this temperature treatment the final crystallisation of the powder appeared what was confirmed by XRD studies. The performed dielectric measurements have revealed the reduction of the dielectric loss of the BFN ceramics compared to materials obtained by classic methods.     

Structure and hard magnetic properties of barium hexaferrite with and without La2O3 prepared by ball milling

The barium hexaferrites have been prepared by ball milling of a BaO₂ and Fe₂O₃ mixture followed by thermal heat treatments. The structure and magnetic properties were investigated using X-ray diffraction, scanning electron microscopy and vibrating sample magnetometer techniques. The effect of grain refiner was also studied and it was found that the hard magnetic properties were improved significantly. The sintered product of barium hexaferrite powders prepared from ball milling has higher coercive force than that of other barium hexaferrite made from oxide/carbonate.     

Na3Zr2-xInxSi2-xP1+xO12系统的组成、结构和离子电导的关系

用固相反应、X射线衍射、金相显微镜观察、测定比热和复平面阻抗谱的方法研究了Na₃Zr_2-xIn_xSi_2-xP_1+xO₁₂系统。在此系统中存在两种固溶体:单斜固溶体(0≤x<0.8)和三方固溶体(0.8≤x≤1.8)。即从x=0.8的组成开始,NASICON型Na₃Zr_2-xIn_xSi_2-x 关键词：     

High-frequency magnetic properties of soft magnetic cores based on nanocrystalline alloy powder prepared by thermal oxidation

FeSiBNbCu nanocrystalline alloy powder was thermally oxidized in an air atmosphere to enhance an oxide layer formation on the surface of the powder and subsequently toroidal shape FeSiBNbCu nanocrystalline alloy powder cores were prepared by compaction at room temperature. The phase change on the surface of FeSiBNbCu nanocrystalline alloy powder by thermal oxidation was analyzed and its effect on the high frequency magnetic properties of the compacted cores was investigated. By thermal oxidation, the formation of the oxide layer consisting of Fe₂O₃, CuO, and SiO₂ on the surface of FeSiBNbCu nanocrystalline alloy powder was enhanced and the thickness of oxide layer could be controlled by changing the thermal oxidation time. FeSiBNbCu nanocrystalline alloy powder core prepared from the powder treated by thermal oxidation exhibits a stable permeability up to high frequency range over 10 MHz. The core loss could be reduced remarkably and the dc-bias property could be improved significantly, which were due to the formation of oxide layer consisting of Fe₂O₃, CuO, and SiO₂ on the FeSiBNbCu nanocrystalline alloy powder. The improvement in high-frequency magnetic properties of the FeSiBNbCu nanocrystalline alloy powder cores could be attributed to the effective electrical insulation by oxide layer between the FeSiBNbCu nanocrystalline alloy powders.     

The gas-sensing potential of nanocrystalline SnO2 produced by a mechanochemical milling via centrifugal action

In this work, the synthesis of undoped nanocrystalline tin dioxide powders and the subsequent preparation of SnO₂ thick-films were studied. An initial mixture of SnCl₂ and Ca(OH)₂ was sealed in a vial for milling in an air atmosphere. Heat treatment of the milled powder resulted in the formation of tetragonal and orthorhombic SnO₂ phases, which was confirmed by X-ray diffraction (XRD) analysis. It was found that crystallite size could be controlled by varying the milling time, the rotation speed and the temperature used for the heat treatment. Crystallite sizes in the range 20 to 30 nm (determined by XRD measurements) were obtained. The total pore volume was 0.22 ml/g for a measured particle size of 37 m²/g. No contamination of the powder during milling was found. The response of the prepared thick-films to H₂S gas in the concentration range 0.5 to 10 ppm in air was investigated as a function of the preparation conditions. The advantage of mechanochemical synthesis of powder is its relative simplicity, low cost and possibility of obtaining isolated, unagglomerated nanosized grains. It is shown that chemical reactions, which usually occur in the vibratory mill to produce the SnO phase, can also be initiated during a short processing time in the centrifugal mill. Received: 25 July 2001 / Accepted: 4 September 2001 / Published online: 20 December 2001     

Intrinsic magnetic properties of In2O3 and transition metal-doped-In2O3

This paper reports on the influence of the sintering temperature and atmosphere and transition-metal doping on the magnetic properties of nanocrystalline and bulk In₂O₃. Undoped nanocrystalline In₂O₃ is diamagnetic whatever the sintering temperature and atmosphere. All single-phase transition-metal-doped In₂O₃ samples are paramagnetic, with a paramagnetic effective moment originating from weakly interacting transition metal ions. No trace of ferromagnetism has been detected even with samples sintered under argon, except extrinsic ferromagnetism for samples with magnetic dopant concentrations exceeding the solubility limit.     

Diversity of TiO<Subscript>2</Subscript> nanopowders’ characteristics relevant to toxicity testing

In this study, the physicochemical properties of several commercial ultrafine TiO₂ powders and their behaviour in the as-received form and colloidal suspensions were analysed. Besides the particle size, the morphology and agglomeration state of the dry powders, dispersibility, ζ-potential and sedimentation in water and in phosphate-buffered saline (PBS) were studied. Also, leaching of ions from the powders during ageing in physiological solution and the ability of the photoactivated powders to decompose organic substances were evaluated. The examined TiO₂ powders revealed diversified characteristics when dispersed in water. In general, while in dry conditions the particle size appeared in the nano-range (down to 32 nm), the particles were agglomerated in aqueous suspensions at pH ~7 and only a minor amount showed dimensions below 200 nm, but none below 100 nm. The inherent pH of the 3 % suspensions varies from 3.7 to 7.5 and the surface charge at these pH values varied from highly positive to highly negative values. In PBS, the surface charge is negative and relatively low for all the samples, which resulted in agglomeration. Five out of six powders exhibited significant photocatalytic activity when exposed to UV irradiation. This also includes one cosmetic-grade powder. Furthermore, during the immersion in aqueous media at physiological temperature, the powders released foreign ions, which might also contribute to the results of cytotoxicity tests. The results revealed the major role of the particle surface charge and its impact on particle dispersion or agglomeration. Due to the high ionic strength in the liquids relevant for cell-surface interaction tests, for all the examined titania powders the nanoparticulate character was lost. However, the presence of impurities and photocatalysis might further contribute to the results of cytotoxicity tests.     

A structural,magnetic and microwave study on mechanically milled Fe-based alloy powders

Fe₉₀M₁₀ powders with M=Fe, Co, Ni, Si, Al, Gd, Dy and Nd were prepared by mechanical milling. Their structure and magnetic properties were investigated. Microwave measurements were performed on the mechanically milled Fe₉₀M₁₀ powders. The results were compared with those of carbonyl Fe powders and coarse Fe powder. It has been shown that fine nanocrystalline Fe-based alloy powders prepared by mechanical milling are promising for microwave applications.     

The formation of a carbonato complex during the mechanochemical treatment of a Na2CO3–Nb2O5 mixture

We have studied the mechanism of the mechanochemical interaction between Na₂CO₃ and Nb₂O₅ in the initial stage of milling, before the final crystalline oxide product, i.e., NaNbO₃, is formed. In order to do this, the milling parameters were chosen to give a relatively low ball-impact energy. In addition, one of the reactants, i.e., Na₂CO₃, was milled also individually so that the mechanochemical effect on the separately milled compound could be determined. X-ray diffraction analysis, thermal analysis and infrared spectroscopy were used in order to characterize the samples after different milling times. The mechanochemical treatment of the Na₂CO₃–Nb₂O₅ mixture results in the formation of a carbonato complex, which represents an intermediate stage in the mechanochemical synthesis of NaNbO₃.