Elaboration of nanostructured Eu3+-doped Gd2O3 phosphor fine spherical powders using polyol-mediated synthesis

Red-emitting Eu³⁺-doped Gd₂O₃ spherical powders were directly precipitated using a polyol method. The as-synthesized powders consist of agglomerates with a spherical shape and a size ranging between 0.4 and 0.6 μm. Each agglomerate is nanostructured and consists of a packing of nanocrystallites (3–5 nm) of a bcc oxide phase whose luminescence presents original features in comparison with bulk materials. The powders were further calcinated and the size of both crystallites and agglomerates, the crystalline structure and the luminescence were studied as a function of the annealing temperature. For temperatures lower than 900 °C, the samples obtained are highly crystalline and possess the classical Eu³⁺ red luminescence. For optimized temperature, the morphology of the particles can be preserved leading to spherical, dense, luminescent and almost monodisperse oxide powders, 0.5 μm in size. PACS 81.07.Bc; 81.07.Wx; 81.16.Be; 75.50.-y; 42.70.-a     

Comparative study of morphology and surface composition of Al–Cr–Fe alloy powders produced by water and gas atomisation technologies

The morphology and surface composition of Al–Cr–Fe alloy powders of 0–63 and 63–100 μm size fractions, produced by gas and water atomisation, have been studied by scanning electron microscopy and Auger electron spectroscopy. While gas atomised particles are of spherical shape, water atomised powders are usually irregular in shape with a complex branched relief. The morphology and composition of surface oxides have been estimated. The surface oxide film is composed of aluminium oxides/hydroxides and contains no Fe and Cr atoms. Two to five water molecules are associated with one Al₂O₃ molecule on the surface of powders. The surface oxide film has a non-uniform thickness, with thick oxide islands separated by thinner oxide film. The parameters of the surface film morphology, such as the island coverage, the oxygen content and the thin film thickness, depend on the atomisation technology used and powder size fraction. Heavily and weakly oxidised powder groups present in all powder fractions are distinguished by Auger spectra analysis. Relationships between heavily and weakly oxidised powder groups are discussed as a function of atomisation technology and size fraction.     

Determination of Particle Size Distribution by Laser Diffraction of Doped‐CeO2 Powder Suspensions: Effect of Suspension Stability and Sonication

The particle size distributions (PSDs) of metal oxide powders are often determined by analyzing suspensions of powders using laser diffraction (e.g. Malvern MasterSizer 2000). Particle agglomeration can effectively bias the resulting distribution towards “unrealistic” particle sizes. Solutions to avoid this problem must be found if a particle distribution based on the elemental or primary particle sizes is desired. In this work, the particle size distribution of doped‐CeO₂ powders was studied. These powders show a crystalline single phase structure of controlled stoichiometry as determined by X‐ray diffraction and ICP analysis. The apparent size distribution was found to be a strong function of suspension stability. Dispersant agents (PBTCA and phosphonoacetic acid) and suspension pH affected stability as characterized by zeta potential measurements. Sonication of the suspensions further enhanced particle de‐agglomeration. Finally, only the combined effect of a dispersant agent, pH adjustment of the suspension and sonication provided a primary particle size distribution. The results presented in this work can be used in the analysis of similar ceramic powders in which strong particle agglomeration is present.     

Comparison of different NaGdF4:Eu synthesis routes and their influence on its structural and luminescent properties

Eu³⁺:NaGdF₄ samples were obtained via co-precipitation in aqueous solution (CP), reversed micelle (RM) method, reaction between solid GdF₃ and NaF solution (SR) as well as a solid-state reaction at high temperatures (SS). The synthesised materials were characterised using X-ray powder diffractometry, TEM microscopy, infrared spectroscopy and TGA analysis. For discussion of optical properties excitation and emission spectra were recorded and emission decay times were measured. The CP and RM methods allow to obtain powders with crystallite size of ∼10 nm, which may be smoothly increased to about 1 μm during post-fabrication heat treatment. Differences in structural and especially in optical properties of phosphors prepared by different techniques are emphasised and applicability of wet-chemistry routes for synthesis of fluoride powders is argued.     

Domain Plotting as a Technique for Summarizing Fineparticle Shape,Texture and Size Information

Increasingly technologists are carrying out measurements on the shape and size distributions of powder grains. Various graphical methods of varying utility have been described in the scientific literature. The use of these methods to summarize information on shape population are illustrated using data from four metal powders. In particular the use of a new graphical method for summarizing information on shape and size using a technique known as domain plotting is illustrated with new data for the powders.     

Preparation of nanostructured manganese-doped biphasic calcium phosphate powders via sol–gel method

Biphasic calcium phosphate ceramics doped with manganese (Mn-doped BCP) were prepared by using chemical doping via sol–gel technique. Four different concentrations of manganese (2, 5, 10, and 15 mol%) have been successfully incorporated into biphasic calcium phosphate (BCP) phases. X-ray diffraction analysis revealed that the phases present in the Mn-doped BCP powders are hydroxyapatite and β-tricalcium phosphate. The Mn-doped powders are more crystalline than Mn-free BCP powder as its crystallinity increased with increasing Mn content. Fourier transform infrared spectrum corresponded to this result as the peak resolutions of PO₄ bands are viewed with more intensity with the increased Mn. Particle size analysis resulted in nanoscale particles for the Mn-doped and Mn-free BCP powders. From field emission scanning electron microscope observation, Mn-doped BCP powders showed nanoscale individual particles but tightly agglomerated into microscale aggregates due to progressive fusion of particles. Hence, it can be concluded that Mn acts as calcination additives of the BCP powders.     

Properties of oxide nanopowders prepared by target evaporation with a pulse-periodic co2 laser

The design and characteristics of a setup for producing metal oxide nanopowders with an output of up to 20 g/h are discussed. The grain mean size in the powders is 15 nm, and the radiation power consumption is 30—40 (W h)/g. Y₂O₃-stabilized ZrO₂ (YSZ) and Al₂O₃ + YSZ nanopowders are prepared by target evaporation with a pulse-periodic CO₂ laser, followed by vapor condensation in an air stream. The mean power, peak power, and efficiency of the pulse-periodic CO₂ laser, excited by a combined discharge, are, respectively, 1 kW, 10 kW, and ≈10%. Data for the powder specific surface, grain shape, and grain size distribution, as well as results of X-ray phase and structure analysis, are reported.     

Magnetic properties of BiFe<Subscript>0.93</Subscript>Mn<Subscript>0.07</Subscript>O<Subscript>3</Subscript> powders obtained by ultrasonic spray pyrolysis

Samples of BiFe_0.93Mn_0.07O₃ with different specific surface area were synthesized for the first time by ultrasonic spray pyrolysis. The resulting powders consist of porous particles of a spherical shape of medium size ~0.5 μm and have record values of residual magnetization and coercive force. It is found that the magnetic properties of the porous powder particles are determined by the distortion of the crystal lattice and the presence of uncompensated magnetic moments of iron ions on the surface.     

Crystal Shape Characterisation of Dry Samples using Microscopic and Dynamic Image Analysis

A standard method to determine particle shape and size is by image analysis. This paper addresses microscopic image analysis (semi‐automated) investigations of two different organic crystalline chemicals generated by batch cooling crystallisation. The results generated from microscopic image analysis were compared with data obtained by dynamic image analysis (automated) because very few contributions are available in the open literature. The chemical systems were polymorphic L‐glutamic acid which crystallises into α (prismatic) or β (needle) form and the non‐polymorphic mono sodium glutamate which crystallises into needles. The images from these techniques were processed to generate information on crystal shape and size. It has been observed that shape effects can distort the size obtained in size characterization studies. In this study, comparisons were made of processing time, number of crystals and accuracy between microscopic and dynamic image analysis. For representative microscopic image analysis, 5000 crystals were analysed in an average of eight hours while several hundred thousand crystals were processed using dynamic image analysis within 15 minutes. Using the parameters D₁₀, D₅₀, D₉₀, span and aspect ratio for statistical comparison, it was found that the results obtained for D₅₀ by the two techniques were comparable and in accordance with other measurements (laser diffraction spectroscopy and ultrasonic attenuation spectroscopy) even though these non‐spherical particles had different orientations during measurement by the two methods. However, substantial differences in span of the distribution and aspect ratio were returned by the two techniques.     

Effect of small particle sizes on the measured density of nanocrystalline powders of nonstoichiometric tantalum carbide TaC<Subscript> <Emphasis Type="Italic">y</Emphasis> </Subscript>

Nanocrystalline powders of the nonstoichiometric tantalum carbide TaC_y (0.81 ≤ y ≤ 0.96) with an average particle size in the range from 45 to 20 nm have been prepared using high-energy ball milling of coarse-grained powders. The density of the initial coarse-grained and prepared nanocrystalline powders of TaC_y has been measured by helium pycnometry. The sizes of particles in tantalum carbide powders have been estimated using the X-ray diffraction analysis and the Brunauer–Emmett–Teller (BET) method. The density of TaC_y nanopowders measured by helium pycnometry is underestimated as compared to the true density due to the adsorption of helium by the highly developed surface of the nanocrystalline powders. It has been shown that the difference between the true and measured densities is proportional to the specific surface area or is inversely proportional to the average particle size of the nanopowders. The large difference between the true and measured pycnometric densities indicates a superhydrophobicity of the tantalum carbide nanopowders.     

Particle analysis and properties of mechanically alloyed Nd16Fe76−xTixB8

Nanocrystalline Nd₁₆Fe_76−xTi_xB₈ hard magnetic powders were prepared by mechanical alloying and respective heat treatment at 973–1073 K /30–60 min. The nanocrystalline hard magnetic powders were investigated by the NanoSight Halo LM10^TM Nanoparticle Analysis System, AFM, SEM and Mössbauer spectrometry. The nanocrystals have average size of 40 nm and the crystals form agglomerates with an average size of about 180 nm. Halo^TM, AFM and SEM techniques are the complementary methods, which give comparable results.     

Microstructural characterization of nanosized YMnO<Subscript>3</Subscript> powders: the size effect

Nanosized YMnO₃ powders of grain size below 100 nm were prepared and characterized by various techniques to understand the size effect of the microstructures and the stoichiometry of the as-prepared powders. It is revealed that the grain surface oxidization state depends on grain size, with a lower oxidization state for the smaller grains. Raman and infrared spectroscopy studies show a remarkable broadening of the reflection peaks with decreasing grain size, identifying the significant surface structure relaxation effect for small-size powders. The ferromagnetic–antiferromagnetic transition around 65–68 °C is determined by the specific-heat measurement, also indicating the remarkable size dependence. PACS 75.47.Lx; 81.07.Wx; 74.62.Yb     

Finite size effects on the structural and magnetic properties of sol–gel synthesized NiFe2O4 powders

Nanoparticles of nickel ferrite have been synthesized by the sol–gel method and the effect of grain size on its structural and magnetic properties have been studied in detail. X-ray diffraction (XRD) studies revealed that all the samples are single phasic possessing the inverse spinel structure. Grain size of the sol–gel synthesized powders has been determined from the XRD data and the strain graph. A grain size of 9 nm was observed for the as prepared powders of NiFe₂O₄ obtained through the sol–gel method. It was also observed that strain was induced during the firing process. Magnetization measurements have been carried out on all the samples prepared in the present series. It was found that the specific magnetization of the nanosized NiFe₂O₄ powders was lower than that of the corresponding coarse-grained counterparts and decreased with a decrease in grain size. The coercivity of the sol–gel synthesized NiFe₂O₄ nanoparticles attained a maximum value when the grain size was 15 nm and then decreased as the grain size was increased further.     

Eu-doped B2O3–ZnO–PbO glass phosphor powders with?spherical shape and fine size prepared by spray pyrolysis

Eu-doped B₂O₃–ZnO–PbO glass phosphor powders with spherical shape and fine size were directly prepared by spray pyrolysis. The glass phosphor powders prepared at a temperature of 1100°C had broad XRD peak at around 28°. One glass phosphor powder was formed from one droplet at the preparation temperature range from 900 to 1100°C. The mean size of the glass phosphor powders was 0.75 μm. The glass transition temperature (T _g ) of the glass phosphor powders prepared by spray pyrolysis was 378.5°C. The excitation spectrum of the glass phosphor powders prepared at the optimum preparation temperature of 1100°C had bands at 362, 381, 392, 463, 525, and 532 nm. The glass phosphor powders had emission spectra with bands at 579, 614, and 653 nm. The glass phosphor powders with doping concentration of Eu of 7 wt% had the maximum photoluminescence intensity. The glass phosphor layer formed from the glass phosphor powders had high transparencies above 90%.     

Sol–gel synthesis and characterization of BaTi2O5 powders

BaTi₂O₅ powders were synthesized using a sol–gel method and characterized. Phase evolution and characteristics of the samples were studied using differential scanning calorimetry, thermogravimetric analysis and X-ray diffractometry. The results show that the formation of BaTi₂O₅ starts around 800 °C and continues until 1200 °C in air, and in this calcination temperature range all powders appear to be single-phase monoclinic BaTi₂O₅. The mechanism of BaTi₂O₅ formation was studied by Fourier-transform infrared spectroscopy. The infrared absorption peaks confirm the existence of a substitution reaction with the chelating reagent corresponding to different modes of vibration characteristic of the acetate group. Scanning electron microscopy observations reveal that the size of the particles that are nearly round in shape formed due to agglomeration is about 0.1–1.5 μm and increases with calcination temperature. PACS 81.70.Pg; 81.20.Fw; 77.84.Dy; 61.10.Nz; 33.20.Ea     

Synthesis of ultrafine single crystals and nanostructured coatings of indium oxide from solution precursor

Indium oxide (In₂O₃) has been widely used in sensors, solar cells and microelectronics. There are several techniques available for making In₂O₃ such as vapor, electrochemical and atomic layer deposition, which are not only expensive but also time consuming processes. In this study, an inexpensive and straightforward synthesis approach is being presented to make micron/submicron size single crystals as well as nanostructured adherent coatings of In₂O₃ using Indium Chloride (InCl₃) powders and InCl₃ solution precursor. Both the powders and the solution precursor were calcined in a furnace to obtain the crystals; however, the liquid precursor was also treated by a DC plasma jet to obtain the nanostructured coatings. The phase transformations during thermal decomposition of InCl₃ powders and solution precursor were investigated via differential scanning calorimetry studies. The phase structure and crystallinity of the crystals and coatings were confirmed by X-ray diffraction. Microstructural characterization of the crystals and coatings was done by scanning electron microscopy, transmission electron microscopy and atomic force microscopy techniques. Size of the crystals was observed to be dependent on the heating schemes adapted during calcination. Solution precursor plasma sprayed In₂O₃ coatings showed porosity and ultrafine particulates with grain size ranging between 10 and 75 nm. Resistivity was determined to be ∼0.553 ± 0.337 kΩ cm. Optical transmittance of In₂O₃ coatings was ∼60-78% in the visible region and it was observed to decrease with increasing the number passes or the thickness of the coatings. Based on the optical transmission data, direct band gap of 3.57 eV was determined.     

From ceramic–matrix nanocomposites to the synthesis of carbon nanotubes

The selective reduction in H₂ of oxide solid solutions produces nanocomposite powders in which transition metal nanoparticles are dispersed inside and on the surface of the oxide matrix grains. When using a H₂/CH₄ reducing atmosphere, the metal nanoparticles that form on the surface of the oxide grains act as catalysts for the CH₄ decomposition and, because of their small size at high temperatures (>800^○C), favor the in-situ nucleation and growth of single-wall and thin multiwall carbon nanotubes. This article reviews our results on the synthesis and characterization of M-MgAl₂O₄ (M=Fe, Fe/Co, Fe/Ni) nanocomposite powders, without and with carbon nanotubes, emphasizing the information that can be derived from Mössbauer spectroscopy as a complement to other characterization techniques.     

Structural and cathodoluminiscent properties of Zr0.95Ce0.05O2 nanopowders prepared by sol-gel and template methods

Nanopowders of Zr_0.95Ce_0.05O₂ composition have been prepared by a standard Pechini-type sol-gel process and by means of a colloidal crystal template approach. In the latter method, inverse opal Zr_0.95Ce_0.05O₂ powders were fabricated employing poly(methyl methacrylate) (PMMA) colloidal crystals as a template. The effects of the two different synthesis routes on the structure and microstructural characteristics of the prepared nanopowders were evaluated by X-ray diffraction and scanning electron microscopy. For both preparation routes, the X-ray diffraction analysis has shown that a tetragonal fluorite structure is formed with a crystallite size of ∼35-40 nm. The scanning electron microscopy measurements indicate that the powder obtained by the sol-gel Pechini-type process is comprised of nanoparticles that are arranged in agglomerates with shape and size relatively uniform whereas the inverse opal Zr_0.95Ce_0.05O₂ nanopowders exhibit the formation of macropores with a mean size of ∼100 nm. The cathodoluminescence spectra of the prepared Zr_0.95Ce_0.05O₂ nanomaterials have been measured in the 300-800 nm wavelength range. The powder prepared by sol-gel method yields a broad emission band centered at 482 nm whereas the emission from the inverse opal preparation is considerably less intense.