Investigation of the effective parameters on the synthesis of Ni-ferrite nanocrystalline powders by coprecipitation method

In this work, synthesis of nanostructured NiFe₂O₄ powders by coprecipitation method using novel precursors, followed by calcination was investigated for the first time. The synthesis was started by dissolving Fe and Ni chlorides and ammonium ferrous sulfate in deionized water. After stirring, the precipitating agent of NaOH was added to the solution and after 1 h aging, the resultant precipitates were washed with deionized water and acetone. Then the powder was characterized and the effects of different parameters such as molarity of precipitant, calcination temperature and the molar ratio of Ni²⁺ ion to the total iron ions were studied. It is concluded that an increase in molarity of precipitant can decrease the calcination temperature. The best conditions giving a single phase Ni-ferrite were using of 2.2 molar NaOH, calcination temperature of 900 °C and the molar ratio of nickel to iron as 0.5. The crystallite size of resulting ferrites was below 50 nm.     

PbO–B2O3–SiO2 glass powders with spherical shape prepared by spray pyrolysis

PbO–B₂O₃–SiO₂ glass powders were directly prepared using spray pyrolysis. The powders were spherical and fine-sized. One glass particle was obtained from one droplet by melting the precursors inside a hot wall tubular reactor. The characteristics of these powders were compared with those of the commercial product, which was prepared using the conventional melting process. The spherical powders, which were prepared using spray pyrolysis at 1000 °C, had broad peaks at around 28° in the XRD spectrum. The glass phase was formed during the spray pyrolysis process even within a short residence time of the powders inside the hot wall tubular reactor. The mean size of the prepared glass powders was 1 μm. The dielectric layers formed from the spherical, fine-sized glass powders had a high transparency at firing temperatures above 520 °C. The maximum transparency of the dielectric layer formed from the glass powders obtained from spray pyrolysis was 95.3% at the firing temperature of 560 °C. The dielectric layer formed from the spherical, fine-sized glass powders had a smooth surface and no void inside the dielectric layer.     

Synthesis and X-ray structural characterization of NiO nanoparticles obtained through gelatin

Nanoparticles of fcc-NiO phase were obtained by heating the dried resin resultant of a mixture of gelatin and NiCl₂ · 6H₂O in aqueous solution. The average particle size and microstrain were calculated from the line broadening of X-ray powder diffraction peaks, and these values were between 15 nm and 78 nm, and 0.056% and 0.172%, respectively. The Rietveld refinement method was applied to all diffraction patterns. The particle size, obtained from this procedure, changes as a function of temperature, heating time and the remarkable reduction due to the addition of NaOH to the solution, which can be attributed to the presence of NaCl crystals and carbon encapsulating NiO nanoparticles during the heating. The heating temperature was in the range of 350–700 °C. Thermo-gravimetric analysis showed that the majority of organic fraction starts to disappear after 300 °C.     

Morphology and crystal structure of A1-doped TiO2 nanoparticles synthesized by vapor phase oxidation of titanium tetrachloride

Al-doped titanium dioxide nanoparticles with precisely controlled characteristics were synthesized in an aerosol reactor between 900 °C and 1500 °C by vapor-phase oxidation of titanium tetrachloride. The effect of process variables (reactor temperature, initial TiCl₄ concentration, residence time and feeding temperature of oxygen) on particle morphology and phase characteristics was investigated using TEM, XRD, EDS, ICP and XPS, etc. The average particle size increased with decreasing oxygen feeding temperature and increasing reaction temperature, residence time and TiCl₄ concentration. The presence of aluminum during gas phase reaction increased the rate of phase transformation from anatase to rutile and altered the particle morphology from polyhedral to irregular crystals. TiO₂ and Al₂O₃ co-precipitated during particle formation which lead to the aluminum solid solution in titania. α-Al₂O₃ and Al₂TiO₅ were observed at AlCl₃/TiCl₄ ratios higher than 1.1 and reactor temperatures in excess of 1400 °C. The rutile content, which increased with increasing Al/Ti ratio and residence time, was at a maximum at about 1200 °C and decreased at both lower and higher reactor temperatures.     

Sintering of nanoparticles of α-Fe2O3 using gelatin

We have demonstrated in this work that single phase of α-Fe₂O₃ nanoparticle can be prepared using gelatin. It was characterized by X-ray powder diffraction (XRPD) technique, gas sorption technique (BET), thermogravimetric analysis (TGA) and transmission electron microscopy (TEM). The nanoparticle was obtained from an aqueous solution of gelatin and FeCl₃ · 6H₂O, as Fe source, annealed in the temperature range of 200–600 °C for 12 h. The mean particle size of this powder measured with XRPD is about 100 nm and this size agrees well with TEM experiments. Also, the TEM result shows agglomeration that was confirmed by BET technique. These results show that gelatin could be an alternative organic precursor to produce metal oxide powders with nanometer dimensions.     

Structural and optical spectroscopy of LiNbO3:Tm nanocrystals embedded in a SiO2 glass matrix

Transparent SiO₂:Li₂O:Nb₂O₅ glass doped with Tm³⁺ has been prepared by the sol–gel method, and heat-treated in air (HT) at temperatures between 500 and 800 °C. X-ray diffraction (XRD) patterns and Raman spectroscopy show SiO₂ and LiNbO₃ phases in samples HT above 650 °C, and a NbTmO₄ phase for T > 750 °C. The XRD SEM analysis show increasing particle size and number with the increase of HT temperature. Intra-4f¹² transitions due to Tm³⁺ ion dispersed in the matrix are observed in samples with T > 650 °C. The luminescence is dominated by the ¹G₄ → ³F₄ (～650 nm), ¹D₂ → ³F₃ (～780 nm), ³H₄ → ³H₆ (～800 nm), ³H₅ → ³H₆ (～1200 nm) and ³H₄ → ³F₄ (～1500 nm) transitions under resonant excitation to the ion levels.     

Structural and magnetic characterization of composite YCo5/α-Fe (20 wt%) powders prepared by mechanical alloying and subsequent annealing

Powder mixtures of YCo₅ + α-Fe (20 wt%) were prepared by high energy mechanical alloying under Ar using a SPEX 8000 mill and subsequent vacuum annealing. The structural and magnetic properties were investigated by X-ray diffraction, DSC, and magnetic measurements. After 4 h of milling, the alloyed powders were found to be composed of an amorphous Y–Co phase and α-Fe with a mean grain size strongly reduced. The DSC curves exhibited an irreversible broad exothermic peak with a maximum at 555 °C associated with the crystallization process. Subsequent high vacuum annealing in the temperature range 550–750 °C led to the formation of rhombohedral Th₂Zn₁₇-type and α-Fe(Co) phases. Samples were magnetically soft showing low remanence and room temperature coercivity in the range 470–800 Oe. The latter is in agreement with the low magnetocrystalline anisotropy along the basal plane exhibited by the Y₂(Co, Fe)₁₇ phase.     

The role of surfactant in synthesis of magnetic nanocrystalline powder of NiFe2O4 by sol–gel auto-combustion method

In this work, a new sol–gel auto-combustion method has been performed to synthesize single phase nickel ferrite nanocrystalline powders by using n-cetyltrimethylammonium bromide, as a cationic surfactant. The gels were prepared from ferric and nickel nitrates and citric acid. Ammonia was used as pH adjusting agent as well. The effects of the surfactant on the after combustion calcination process and the reduction of the resulting powder crystallite size which affects the magnetic properties of the material were investigated by XRD and DTA/TGA techniques. The results showed that the ignition of the gels in air have a self-propagating behavior. Addition of surfactant to the starting solution affected the crystallite size of the synthesized powders and their phase constitution. The crystallite size of nickel ferrite powder in sample with surfactant was obtained 31.2 nm. The other important result of this study was production of single phase nickel ferrite directly after combustion while without surfactant the nickel ferrite single phase was obtained only after a post-calcination process at 1000 °C.     

Synthesis of BaCeO3 powders by a fast aqueous citrate–nitrate process

A fast aqueous citrate–nitrate process has been successfully used to prepare the stoichiometric BaCeO₃ powders. It was found that an optimal amount of water added in the mixing process of barium nitrate, cerium nitrate, and citric acid is helpful on the formation of a clear gel that was subsequently formed by the condensation of added diethylene glycol monomers at room temperature. The gel was heated to an easily handled precursor in a powder form at 400 °C for 1 h. The precursors were characterized by X-ray powder diffraction (XRD), differential thermal analysis (DTA), thermal gravimetric analysis (TGA), thermal mechanical analysis (TMA), Fourier transform infrared spectrometry (FTIR), and scanning electron microscopy (SEM). The phase evolution from the precursor to the desired BaCeO₃ powders by XRD and FTIR revealed that the presence of BaCO₃ is a complex problem in this process. A high-temperature trigonal BaCO₃ intermediate was found between 800 °C and 1000 °C by the in-situ X-ray powder diffraction experiments. The phase purity of the formed BaCeO₃ powders may be more properly identified by FTIR instead of XRD because of the amorphous feature of the carbonate phase. The TMA revealed that at 1100 °C part of the precursor crystallized to needle-shaped BaCeO₃ crystals accompanied by a small expansion, which is also related to the presence of BaCO₃ in the precursor, and this can be removed by a preheating treatment of the precursor at 900 °C for 4 h.     

Ultrafine Co1−xZnxFe2O4 particles synthesized by hydrolysis: Effect of thermal treatment and its relationship with magnetic properties

Co_1−xZn_xFe₂O₄ (x = 0, 0.2 and 0.4) fine powders with particles size of 3 nm were prepared by hydrolysis method. The powders were annealed at 500 °C for 3 h. With heat treatment, the average particles size increased to 12 nm with corresponding increase in blocking temperature, saturation magnetization and reduced remanence. A significant increase in coercive field was found only for the pure CoFe₂O₄.     

Fabrication and characterization of transparent conductive ZnO:Al thin films prepared by direct current magnetron sputtering with highly conductive ZnO(ZnAl2O4) ceramic target

Using a highly conductive ZnO(ZnAl₂O₄) ceramic target, c-axis-oriented transparent conductive ZnO:Al₂O₃ (ZAO) thin films were prepared on glass sheet substrates by direct current planar magnetron sputtering. The structural, electrical and optical properties of the films (deposited at different temperatures and annealed at 400°C in vacuum) were characterized with several techniques. The experimental results show that the electrical resistivity of films deposited at 320°C is 2.67×10⁻⁴ Ω cm and can be further reduced to as low as 1.5×10⁻⁴ Ω cm by annealing at 400°C for 2 h in a vacuum pressure of 10⁻⁵ Torr. ZAO thin films deposited at room temperature have flaky crystallites with an average grain size of ∼100 nm; however those deposited at 320°C have tetrahedron grains with an average grain size of ∼150 nm. By increasing the deposition temperature or the post-deposition vacuum annealing, the carrier concentration of ZAO thin films increases, and the absorption edge in the transmission spectra shifts toward the shorter wavelength side (blue shift).     

Comparative study on photoluminescence of amorphous and nano-crystalline YAG:Tb phosphors prepared by a combustion method

Amorphous and nano-crystalline Y₃Al₅O₁₂:Tb phosphor samples were obtained via a facile combustion method by calcination at various temperatures, using yttrium oxide and aluminum nitrite as starting materials and citric acid as fuel. XRD, FT-IR and TEM results showed that the products were amorphous if prepared at 750 °C, well-crystalline when treated above 850 °C. In addition, partially crystalline YAG phase was observed at 800 °C (in air). The excitation spectra of the samples calcined at 750 °C and 800 °C exhibited some difference in the 230–255 nm range in comparison to those of nano-crystalline YAG:Tb, i.e. an extra band centered at 250 nm was detected via Gaussian curve-fitting. Furthermore, the photoluminescence intensity of as-synthesized samples decreased obviously with increasing the crystallinity under 250 nm excitation. Contrary, it increased monotonously when altering the excitation wavelength to 323 nm. The concentration-dependent emission spectra of samples calcined at 800 °C revealed that the strongest intensity could be obtained with 10% Tb doping. Red-shifts indicated changes of the inter-atomic distances within the Tb³⁺ coordination polyhedron with increasing Tb concentration. The low temperature photoluminescence of partially crystalline YAG:10% Tb was also investigated, displaying good-resolution but reduced intensity compared to the room-temperature photoluminescence.     

Effect of the surfactant nature on the thermo-stability of surface modified SnO2 nanoparticles

The modification of particle surface properties by the addition of small surfactant molecules in the initial sol is one strategy to minimize the strong tendency to aggregation and coarsening of nanoparticles prepared from the sol–gel process. In this work, the effect of the nature of the surfactant, Tiron^® ((OH)₂C₆H₂(SO₃Na)₂ · H₂O, anionic) or Catechol^® (C₆H₄-1,2-(OH)₂, non-ionic) or Maptac^® ([N(CH₃)₃(CH₂)₃NHCOC(CH₂CH₃)]⁺Cl^?, cationic), grafted on the SnO₂ nanoparticles on the mesoporosity of powders fired at 600 °C is presented. SnO₂ powders were prepared from an one-pot sol–gel route in which the hydrolysis of SnCl₄ · 5H₂O in aqueous solution was carried out in presence of the surfactant. The Fourier transform infrared (FTIR) spectroscopy and gravimetric and differential thermo-analysis (TG/DTA) results show that the thermo-stability of surface grafted SnO₂ nanoparticles obeys the following series: Tiron^® > Catechol^® > Maptac^®. The N₂ adsorption isotherms results evidence that the mesopores texture (specific surface area, pore volume and average pore size) can be tuned in a controlled way by increasing the amounts of Tiron^® or Catechol^® molecules grafted on the surface of SnO₂ nanoparticles.     

Synthesis and thermal behavior of different aluminosilicate gels

In this study a series of different gels (inorganic polymers) was synthesized with a single procedure, although varying the pH of the synthesis medium across a wide range of values. The chemicals used were as follows: a 0.25 M solution of K₂O · SiO₂ · 9H₂O as the source of silica, a 0.25 M solution of Al(NO₃)₃ · 9H₂O as the source of aluminum; and a 0.25 M solution of KOH to regulate the pH. Gel thermal stability was also determined, up to 1200 °C. The results show that at acidic pH both Si-rich and Al-rich gels are formed, which may crystallize into cristoballite and mullite, respectively, under thermal treatment. The gels synthesized at alkaline pH, in turn, remain stable at temperatures over 1200 °C. Depending on their composition, such gels may crystallize into leucite.     

Structural,electrical and optical properties of in-situ phosphorous-doped Ge layers

We have studied the impact of temperature and pressure on the structural and electronic properties of Ge:P layers grown with GeH₄+PH₃ on thick Ge buffers, themselves on Si(0 0 1). The maximum phosphorous atomic concentration [P] exponentially decreased as the growth temperature increased, irrespective of pressure (20 Torr, 100 Torr or 250 Torr). The highest values were however achieved at 100 Torr (3.6×10²⁰ cm^?3 at 400 °C, 2.5×10¹⁹ cm^?3 at 600 °C and 10¹⁹ cm^?3 at 750 °C). P atomic depth profiles, “box-like” at 400 °C, became trapezoidal at 600 °C and 750 °C, most likely because of surface segregation. The increase at 100 Torr of [P] with the PH₃ mass-flow, almost linear at 400 °C, saturated quite rapidly at much lower values at 600 °C and 750 °C. Adding PH₃ had however almost no impact on the Ge growth rate (be it at 400 °C or 750 °C). A growth temperature of 400 °C yielded Ge:P layers tensily-strained on the Ge buffers underneath, with a very high concentration of substitutional P atoms (5.4×10²⁰ cm^?3). Such layers were however rough and of rather low crystalline quality in X-ray Diffraction. Ge:P layers grown at 600 °C and 750 °C had the same lattice parameter and smooth surface morphology as the Ge:B buffers underneath, most likely because of lower P atomic concentrations (2.5×10¹⁹ cm^?3 and 10¹⁹ cm^?3, respectively). Four point probe measurements showed that almost all P atoms were electrically active at 600 °C and 750 °C (1/4th at 400 °C). Finally, room temperature photoluminescence measurements confirmed that high temperature Ge:P layers were of high optical quality, with a direct bandgap peak either slightly less intense (750 °C) or more intense (600 °C) than similar thickness intrinsic Ge layers. In contrast, highly phosphorous-doped Ge layers grown at 400 °C were of poor optical quality, in line with structural and electrical results.     

Hydrothermal synthesis of BaTiO3 thin films on nanoporous TiO2 covered Ti substrates

It is well known that there is an upper limit (<0.25 μm) for the thickness of hydrothermal thin films grown on Ti substrate in the 100–200 °C temperature range, even the reaction time is extended to several weeks. In this paper, BaTiO₃ thin films have been firstly hydrothermally synthesized on titanium substrates covered with a nanoporous TiO₂ layer. By using TiO₂ covered substrates, the thickness of BaTiO₃ films can easily reach ∼1.0 μm at 110 °C after only 2 h hydrothermal treatment. It is found that the large quantity of pores with size at the tens of nanometer range in the oxide layer served as easy paths for the diffusion of Ba²⁺ and OH⁻ and enabled the film grow thicker. SEM and XRD results show that the films are crack-free and in polycrystalline phase.     

Photoluminescence and microstructure of Mn-doped ZnS phosphor powder co-fired with ZnS nanocrystallites

In this study, we correlated the photoluminescence (PL) with the microstructure of ZnS:Mn phosphor powders prepared by firing ZnS with MnO (1 mol%), NaCl (1 mol%) and ZnS nanocrystallites (NCs) in the range of 0–100 wt% at 600–1000 °C for 2 h in the atmosphere of 3%H₂/Ar. ZnS NCs of 10–30 nm in size were produced by co-precipitation of zinc nitrate and sodium sulfide solutions at room temperature. Thermal analysis (DTA/TG) and X-ray diffraction (XRD) results indicated that the cubic-hexagonal transformation temperature of ZnS NCs was lowered to approximately 600 °C, which was much lower than that of bulk ZnS (1020 °C). PL measurements revealed that ZnS:Mn fired with 1 wt% ZnS NCs showed the optimal luminescence intensity when compared to those without or with higher ZnS NCs (>1 wt%). An appropriate amount of ZnS NCs (1 wt%) acting as the flux in the firing process was inferred to avoid the inhomogeneous distribution of Mn²⁺ as well as the migration of excitation energy to quenching sites and therefore to result in the enhanced PL intensity.     

Effect of preparation conditions on the phase transformation of mesoporous alumina

A post-hydrolysis method is proposed for the easy-and-fast preparation of mesoporous γ-alumina (MA) at room temperature using an alkyl carboxylate as a chemical template. Water was used as an initiating solvent in the hydrolysis of aluminum alkoxide in the terminal reaction procedure. Hydrolysis and condensation reactions were simultaneously induced. The initial phase of as-made MA was bayerite and/or (pseudo)boehmite, and was transformed into active alumina at 250 °C, which remained thermally stable at temperatures up to 600 °C. The phase transformation of alumina was analyzed with DTA, XRD and ²⁷Al MAS NMR. Based on an ²⁷Al MAS NMR analysis, calcined alumina prepared via a hydrothermal method contained a high ratio of Al^VI to Al^IV (∼4.8), which is not fully transformed into active alumina, and aluminum hydroxide (AlOOH or Al(OH)₃) was detected. These results suggest that high temperature is required to obtain an active form of alumina, when the hydrothermal method is used. Another factor is the ratio of Al^VI to Al^IV in the MA, which is also dependent on the pH of the reactant. Pore size, surface area, and pore structure could be controlled by adjusting the molar ratio of water to aluminum precursor, and the preparation temperature. The pore size of MA was adjustable from 2 to 7 nm, and the MA indicated a large surface area of 300 ∼ 500 m²/g.     

Fabrication of Cr4+,Nd3+:YAG transparent ceramics for self-Q-switched laser

Transparent 0.1 at.%Cr,1.0 at.%Nd:YAG (Y₃Al₅O₁₂) ceramics were fabricated by a solid-state reaction and vacuum sintering with CaO as a charge compensator and tetraethyl orthosilicate (TEOS) as a sintering aid using high-purity powders of Al₂O₃, Y₂O₃, Nd₂O₃ and Cr₂O₃. The mixed powder compacts were sintered at 1800 °C for 5 h and 30 h under vacuum. The optical transmittance of the Cr,Nd:YAG ceramics sintered at 1800 °C for 5 h and 30 h is ∼63% and ∼78% in the infrared wavelengths, respectively. The two samples exhibit pore-free structures and the average grain size is about 10 and 20 μm. For the sample sintered at 1800 °C for 5 h, the dominant fracture mechanism is the transgranular fracture. With increase of holding time up to 30 h, the ratio of intergranular fracture surfaces increase and more Cr³⁺ ions in the Cr,Nd:YAG ceramic transform to Cr⁴⁺. High-quality Cr⁴⁺,Nd³⁺:YAG transparent ceramics may be a potential self-Q-switched laser material.     

Investigations on sol–gel process and structural characterization of SiO2-P2O5 powders

The purpose of the study is to investigate the influence of the precursors, pH of the solution and temperature on the gelation and structure evolution of the samples from the SiO₂-P₂O₅ system. Tetraethoxysilane (TEOS) was used as precursor for SiO₂ and triethylphosphate (TEP) or phosphoric acid for P₂O₅, together with water as reagent for hydrolysis reaction and ethylic alcohol as solvent. The pH of the sols was modified by adding hydrochloric acid, in the case of TEP and by adding ammonia, in the case of H₃PO₄. The samples have been prepared starting from P₂O₅/SiO₂ = 1/10 and 1/5 molar ratio, H₂O/TEOS = 1; 2; 3 mass ratios and C₂H₅OH/TEOS = 1 mass ratio. We prepared silico-phosphate samples in the 1.5–5 pH domain and we observed that in all the cases, the lowest gelation time was found in the 3.5–4.5 pH range. We found that for the same pH value samples prepared with H₃PO₄ had a lower gelation time (few days) by comparison with the samples prepared with TEP (weeks), explainable by the low rate of the hydrolysis and condensation reactions of TEP. When the amount of water was increased, the gelation time increased in the case of samples prepared with H₃PO₄ and it was not significantly changed in the case of the samples prepared with TEP. The increasing of the solution temperature up to 40–41 °C yielded a decreasing of the gelation time (hours), especially for the samples prepared with H₃PO₄ by comparison with those prepared using TEP. In all the cases, the increased amount of water resulted in an increasing of the gelation time, even the temperature was raised. FTIR and Raman spectroscopy characterization aimed at getting information about the structural changes in the case of the samples dried in air and also for those heated at 100 °C, 300 °C, 600 °C and 900 °C. Vibration modes specific for SiOEt, SiOH, hydrogen bonds, H₂O and combined vibrations have been observed, which are in agreement with those revealed in literature data. ³¹P and ²⁹Si MAS NMR spectra gave interesting information about first surrounding of P and Si ions meaning the type and proportion of Q species and their evolution starting from the room temperature up to 900 °C.