Comparative study of nanocrystalline Zr0.85Ce0.15O2 powders synthesised by spray-pyrolysis and gel-combustion methods

Zr_0.85Ce_0.15O₂ nanopowders synthesised by gel-combustion and spray-pyrolysis methods were comparatively studied by means of X-ray diffraction, Raman spectroscopy, thermogravimetric and differential thermal analyses, specific surface area measurements, scanning and transmission electron microscopies and chemical analysis. Fully tetragonal powders were obtained by both methods, as determined by X-ray diffraction and Raman spectroscopy. Both materials exhibited extremely small crystallite sizes (about 6 nm) and high specific surface areas (93 m²/g and 42 m²/g for gel-combustion and spray-pyrolysis powders, respectively). In both cases, no tetragonal-to-monoclinic transition was observed in the whole temperature range up to 1300 °C by differential thermal analysis. The amounts of the expected impurities (Si, B, C) were acceptable and comparable in both cases.     

Nanostructured cobalt oxides (Co3O4 and CoO) and metallic Co powders synthesized by the solution combustion method

The combustion synthesis technique using glycine and urea as fuels and cobalt nitrate as an oxidizer is capable of producing well-crystallized Co₃O₄, CoO, as well as metallic Co powders. An interpretation based on the thermodynamic viewpoint and the measurement of the combustion temperatures during the reactions occurring for various fuel-to-oxidant ratios was proposed for a study of the nature of combustion and its correlation with the characteristics of as-synthesized powders. The largest measured specific surface area of the powders was 36 m²/g at a 0.14 glycine-to-nitrate ratio. The crystallites were nano-sized ranging from approximately 23 to 90 nm.     

Alanine-assisted low-temperature combustion synthesis of nanocrystalline LiMn2O4 for lithium-ion batteries

Nanocrystalline LiMn₂O₄ powders have been synthesized by combustion process in a single step using a novel fuel, l-alanine. Thermogravimetric analysis and differential thermal analysis of the gel indicate a sharp combustion at a temperature as low as 149 °C. Quantitative phase analysis of X-ray diffraction data shows about 97% of phase purity in the as-synthesized powder, which on further calcination at 700 °C becomes single phase LiMn₂O₄. High Brunauer, Emmett, and Teller surface area values obtained for ash (53 m²/g) and calcined powder (23 m²/g) indicate the ultrafine nature of the powder. Average crystallite size is found to be ∼60-70 nm from X-ray diffraction analysis and transmission electron microscopy. Fourier transformed infra-red spectrum shows two strong bands at 615 and 511 cm⁻¹ originating from asymmetrical stretching of MnO₆ octahedra. A nominal composition of Li_0.88 Mn₂O₄ is calculated from the inductive coupled plasma analysis. From UV-vis spectroscopy, an optical band gap of 1.43 eV is estimated which is assigned to a transition between t_2g and e_g bands of Mn 3d. Electrochemical charge-discharge profiles show typical LiMn₂O₄ behavior with a specific capacity of 76 mAh/g.     

Properties of TiO2 prepared by acid treatment of BaTiO3

TiO₃ powders were prepared by acid treatment of BaTiO₃ and their properties were investigated. The BaTiO₃ powder was subjected to HNO₃ in concentrations ranging from 10⁻³ to 8 M at 90 °C for 0.5-6 h. Dissolution of BaTiO₃ and precipitation of TiO₂ occurred at acid concentrations of 2-5 M. BaTiO₃ dissolves completely to form a clear solution at reaction times of 0.5-1 h, but a rutile precipitate is formed after 2 h of acid treatment. By contrast, anatase is precipitated by adjusting the pH of the clear solution to 2-3 using NaOH or NH₄OH solution. The rutile crystals were small and rod-shaped, consisting of many small coherent domains connected by grain boundaries with small inclination angles and edge dislocations, giving them a high specific surface area (S_BET). With increasing HNO₃ concentration, the S_BET value increased from 100 to 170 m²/g while the crystallite size decreased from 25 to 11 nm. The anatase crystals obtained here were very small equi-axial particles with a smaller crystallite size than the rutile and S_BET values of about 270 m²/g (higher than the rutile samples). The photocatalytic activity of these TiO₂ was determined from the decomposition rate of Methylene Blue under ultraviolet irradiation. Higher decomposition rates were obtained with larger crystallite sizes resulting from heat treatment. The maximum decomposition rates were obtained in samples heated at 500-600 °C. The photocatalytic activity of the TiO₂ was found to depend more strongly on the sample crystallite size than on S_BET.     

Solid state and solution synthesis of Ba(Mg1/3Ta2/3)O3: A comparative study

Ba(Mg_1/3Ta_2/3)O₃ [BMT] dielectric ceramics are prepared by solid state (one step, two step and molten salt synthesis) and wet chemical methods (precipitation, citrate gel and sol-gel). The formation mechanism of BMT in each synthesis technique is discussed. The formation temperature and particle size of the formed BMT were found to be much lesser (in nanometer range) for solution synthesized powders. It is found that synthesis by sol-gel method resulted in the formation of ultra pure nanopowders of BMT at about 600 °C with average crystallite size of about 18 nm where as in solid state synthesis the formation of BMT was formed at about 1100 °C with average crystallite size of 220 nm. On sintering these powders, densification and grain growth of the chemically derived powders were found to be lower than that of solid state synthesized BMT powder. This has resulted in a slight decrease in density and microwave dielectric properties of the solution synthesized BMT samples. It is found that the microwave dielectric properties improved with increase in the average grain diameter of the sintered BMT ceramics.     

Mixture of fuels approach for the solution combustion synthesis of Al2O3-ZrO2 nanocomposite

A modified solution combustion approach was used for the first time in the preparation of nanosize zirconia toughened alumina (ZTA) composite. ZTA-1 with an average particle size of ∼37 nm was prepared using corresponding metal nitrates and urea. ZTA-2 with an average particle size of <10 nm was prepared by using mixture of fuels such as ammonium acetate, urea and glycine. The products formed were characterised by powder X-ray diffractometry, Transmission electron microscopy and BET surface area analysis. By using mixture of fuels, the energetics of the combustion reaction and eventually the properties of the combustion product have been changed. A series of combustion reactions were carried out to optimise the fuel ratio combinations required to obtain <10 nm ZTA particles. The microstructure of ZTA consisted of crystallites of Al₂O₃ and ZrO₂ both of which were nanocrystalline as evident from TEM.     

Synthesis and characterization of (Na0.85K0.15)0.5Bi0.5TiO3 ceramics by different methods

The (Na_0.85K_0.15)_0.5Bi_0.5TiO₃ (BNKT) powders were synthesized by solid-state method, sol-gel method and stearic acid method. Microstructure, piezoelectric and dielectric properties of the ceramics were investigated. Attempts had been made to understand the reaction processes by using thermo gravimetric (TG) and differential scanning calorimetry (DSC). The BNKT powders have a perovskite structure with average crystallite sizes of 168 nm, 85 nm and 79 nm, corresponding to the solid-state method, the sol-gel method and the stearic acid method, respectively. The ceramics derived from the powder synthesized by sol-gel method presents the most homogeneous microstructure and largest grain size (5-7 μm). The effects of average crystallite size on microstructures and electric properties of the BNKT ceramics were investigated. Both the piezoelectric properties and dielectric properties were enhanced with the increase of grain size.     

Dopant induced variations in microstructure and optical properties of CeO2 nanoparticles

Nanocrystalline CeO₂ particles doped in the range of 0-20% of Ca²⁺, La³⁺, and Zr⁴⁺ have been prepared from hydrothermal synthesis of nitrate solutions at 200 °C and the influences of the dopants on microstructure and optical properties of the nanoparticles have been investigated. The unit cell parameter is found to be modified by −0.39, +0.83 and +0.16% for doping of 20% Zr⁴⁺, La³⁺, and Ca²⁺, respectively. For each batch prepared, nanoparticles with a narrow size distribution of 5-15 nm have been obtained. A high-resolution transmission electron microscopy investigation reveals that these particles are single crystals mostly having hexagonal, square or circular two-dimensional projections. UV-visible spectra of doped powders exhibit shift of the absorption edge and absorption peak with respect to those of the undoped CeO₂ particles and has been attributed to compensation of Ce³⁺ and decreasing crystallite size as result of doping.     

Factors affecting CO oxidation over nanosized Fe2O3

Nanocrystallite iron oxide powders with different crystallite sizes were prepared by co-precipitation route. The prepared powders with crystallite size 75, 100 and 150 nm together with commercial iron oxide (250 nm) were tested for the catalytic oxidation of CO to CO₂. The influence of different factors as crystallite size, catalytic temperature and weight of catalyst on the rate of catalytic reaction was investigated using advanced quadrupole mass gas analyzer system. It can be reported that the rate of conversion of CO to CO₂ increased by increasing catalytic temperature and decreasing crystallite size of the prepared powders. The experimental results show that nanocrystallite iron oxide powders with crystallite size 75 nm can be recommended as a promising catalyst for CO oxidation at 500 °C where 98% of CO is converted to CO₂. The mechanism of the catalytic oxidation reaction was investigated by comparing the CO catalytic oxidation data in the absence and presence of oxygen. The reaction which was found to be first order with respect to CO is probably proceeded by adsorption mechanism where the reactants are adsorbed on the surface of the catalyst with breaking OO bonds, then CO pick up the dissociated O atom forming CO₂.     

Synthesis of alumina powders by the glycine-nitrate combustion process

The combustion synthesis technique using glycine as fuel and aluminum nitrate as an oxidizer is able to produce alumina powders. Thermodynamic modeling of the combustion reaction shows that as the fuel-to-oxidant ratio increases, the amount of gases produced and adiabatic flame temperatures also increases. X-ray diffractions showed the amorphous structure for as-synthesized powder and presence of well-crystallized α-Al₂O₃ after calcination at 1100 °C during soaking time of 1 h. Alumina's largest measured specific surface area was 15 m²/g with BET method and 0.51 glycine-to-nitrate ratio.     

Synthesis and photoluminescence properties of SrLu2O4:Eu superfine phosphor

Superfine powder SrLu₂O₄:Eu³⁺ was synthesized with a precursor prepared by an EDTA - sol-gel method at relatively low temperature using metal nitrate and EDTA as starting materials. The heat decomposition mechanism of the precursor, formation process of SrLu₂O₄:Eu³⁺and the properties of the particles were investigated by thermo-gravimetric (TG) - differential thermal analysis (DTA), X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence (PL) analyses. The results show that pure SrLu₂O₄:Eu³⁺ superfine powder has been produced after the precursor was calcinated at 900 °C for 2 h and has an elliptical shape and an average diameter of 80-100 nm. Upon excitation with 250 nm light, all the SrLu₂O₄:Eu³⁺ powders show red and orange emissions due to the 4f-4f transitions of Eu³⁺ ions. The highest photoluminescence intensity at 610 nm was found at a content of about 6 mol% Eu³⁺. Splitting of the ⁵D₀-⁷F₁ emission transition revealed that the Eu³⁺ ions occupied two nonequivalent sites in the crystallite by substituting Lu³⁺ ions.     

Synthesis and characterization of nanocrystalline MgAl2O4 spinel via sucrose process

Nanocrystalline MgAl₂O₄ spinel powder was synthesized using metal nitrates and a polymer matrix precursor composed of sucrose and polyvinyl alcohol (PVA). The precursor and the calcined powders were characterized by simultaneous thermal analysis (STA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). According to XRD results, the inceptive formation temperature of spinel via this technique was between 600 and 700 °C. The calcined powder at 800 °C for 2 h has faced shaped morphology and its crystallite size is in the range of 8-12 nm. Further studies also showed that the amount of polymeric matrix to metal ions has significant influence on the crystallite size of synthesized magnesium aluminate spinel powder.     

A low-temperature process to synthesize rutile phase TiO2 and mixed phase TiO2 composites

This works employed K₂Ti₄O₉, a novel Ti source, to prepare TiO₂ powders. By a “low-temperature dissolution-reprecipitation process” (LTDRP), rutile phase TiO₂ was successfully synthesized after reacting at 50 °C for 48 h. The obtained sample showed a specific surface area about 45 m²/g, and excellent activity in photo-destruction of NO_x gas. The coupling of rutile phase TiO₂ with commercial anatase TiO₂ showed significant effect in further enhancing the photocatalytic activity.     

Influence of fuels on the morphology of undoped Y3Al5O12 and photoluminescence of Y3Al5O12:Eu prepared by a combustion method

Undoped and Eu-doped yttrium aluminum garnet nano-powders were prepared by a facile combustion method with citric acid/ethylene diamine tetraacetic acid (EDTA) as fuels and nitrates as oxidizers. The precursors and powders calcined at 1030 °C were investigated using thermogravimetric (TG), differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscope (SEM), and Brunauer-Emmett-Teller (BET) surface area measurements. It was found that the powders could be indexed with a garnet structure. The grains were in shape of hemispherical with sizes between 60 nm and 100 nm. With decreasing the citric acid/EDTA ratio, the crystallite size decreased steadily and the specific surface area increased. Investigations of photoluminescence (PL) revealed that as-synthesized YAG:Eu³⁺ phosphor samples exhibited an orange emission band with a main peak at 591 nm under the excitation of 394 nm. As citric acid amounts increased, the quality of crystallinity became higher and the luminescent properties were monotonously enhanced.     

Combustion synthesis of La0.7Sr0.3Co0.5Fe0.5O3 (LSCF) porous materials for application as cathode in IT-SOFC

La_0.7Sr_0.3Co_0.5Fe_0.5O₃ (LSCF) porous materials have attracted a substantial interest for application as cathode in solid oxide fuel cells of intermediate temperature (IT-SOFC). This work investigates the effect of different propellants (urea, glycine, citric acid and sucrose) in the preparation of LSCF powders by the combustion method and also the influence of the sintering temperature on the porosity and electrical conductivity. TGA profiles of the as-prepared samples showed a lower weight loss for the sample prepared with glycine, associated with the higher combustion temperature. XRD patterns presented characteristic reflections of LSFC perovskite and a small formation of secondary phases, with nanometric crystallite sizes (9-20 nm). SEM analysis revealed the loose and porous structure of the powder materials. Densification studies were carried within 950-1100 °C, showing that porosity decreased with increasing sintering temperature. Electrical conductivity was measured in the temperature range 300-800 °C and correlated with the sintering temperature.     

Hydrothermal synthesis of nanocrystalline VO2 from poly(diallyldimethylammonium) chloride and V2O5

Novel vanadium dioxide nanorods were fabricated from V₂O₅ in the presence of a reducing agent, the poly(diallyldimethylammonium chloride) (PDDA) via a hydrothermal method at 180 °C for 48 h. The samples produced were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), infrared spectroscopy (FTIR), nitrogen adsorption (BET) and thermogravimetry (TG/DTG). The nanorods obtained are approximately 50 nm wide and from 300 to 500 nm long and presents high surface area (42 m² g⁻¹). The nanocrystalline B phase VO₂ is not produced by hydrothermal treatment in the absence of the PDDA polyelectrolyte.     

The effect of Tb doping on the structure and spectroscopic properties of MgAl2O4 nanopowders

In this paper, a modified sol-gel method was employed to prepare nanostructured MgAl₂O₄ spinel powders doped with Tb³⁺ ions and thermally treated at 700 and 1000 °C for 3 h. The structural properties of the prepared at 700 and 1000 °C powders where characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). According to obtained XRD patterns the formation of single-phase spinels after calcination was confirmed. The XRD analyses demonstrated that the powders were single-phase spinel nanopowders with high crystallite dispersion. The Rietveld method was applied to calculate lattice parameters. The averaged spinel particle size was determined to be ∼10 nm for calcination at 700 °C and ∼20 nm at 1000 °C. The emission and excitation spectra measured at room and low temperature (77 K) for the samples calcined at 700 and 1000 °C demonstrated characteristic spectra of Tb³⁺ ions. The effect of MgAl₂O₄:Tb³⁺ grain sizes on luminescence properties was noticed.     

A facile method for nickel catalyst immobilization on ultra fine Al2O3 powders

A pure nickel coating has been successfully plated on the surface of ultra fine Al₂O₃ particles via a facile electroless plating method. Coating morphology and crystallite size can be tailored by pH values. Dense coating with the maximum crystallite size of 24 nm was obtained at pH 11.0 and porous coating with the minimum crystallite size of 15 nm was obtained at pH value 12.5. The plated powders have been demonstrated to be an effective catalyst for growing boron nitride nanotubes.     

Modification of the physical properties of semiconducting MgAl2O4 by doping with a binary mixture of Co and Zn ions

The effects of doping of MgAl₂O₄ by a binary mixture of Co and Zn ions on the absorbance, electrical resistivity, capacitance, thermal conductivity, heat capacity and thermal diffusivity are reported in this paper. The materials with the nominal composition Mg_1−2x(Co,Zn)_xAl₂O₄ (x = 0.0-0.5) are synthesized by solution combustion synthesis assisted by microwave irradiation. The substituted spinels are produced with a Scherrer crystallite size of 18-23 nm, as opposed to 45 nm for undoped samples, indicated by X-ray diffraction and confirmed by transmission electron microscopy. These materials also show better thermal stability in the temperature range of 298-1773 K. Three strong absorption bands at 536, 577 and 630 nm are observed for the doped samples which are attributed to the three spin allowed (⁴A₂ (F) → ⁴T₁ (P)) electronic transitions of Co²⁺ at tetrahedral lattice sites while pure magnesium aluminate remains transparent in the whole spectral range. The semiconducting behavior of the materials is evident from the temperature dependence of the electrical resistivity. Resistivity and activation energy are higher for the substituted samples. Fitting of the resistivity data is achieved according to the hopping polaron model of solids. Both dielectric constant and loss increase on account of doping. The dielectric data are explained on the basis of space charge polarization. The thermal conductivity and diffusivity are lowered and the heat capacity is increased in the doped materials. Wiedemann-Franz's law is used to compute the electronic and lattice contributions towards the total thermal conductivity.     

Pretreatment and sintering of Si3N4 powder synthesized by the high-temperature self-propagation method

Self-propagation high-temperature synthesis (SHS) was applied for the synthesis of low-cost Si₃N₄ powder. The powder was purified and ground until its particle size reached submicron levels and its purity reached 98%. Using this pretreated powder, with α/β = 60/40 content, fully dense Si₃N₄ ceramics, having improved mechanical properties, were obtained by liquid-phase sintering in the presence of (Y, La)₂O₃-AlN. The mechanical properties achieved finally were as follows: strength, 784 MPa; hardness, 15.1 GPa; and fracture toughness, 5.2 MPa m^0.5. The behaviors of the SHS-Si₃N₄ powders before and after the pretreatment were compared. The relation between microstructure and mechanical properties of the sintered specimens and the effect of different β content in the powder on the sintering process of Si₃N₄ were also studied.