Synthesis of MgNb2O6 by coprecipitation

A simple coprecipitation technique was used successfully to synthesize fine powders of MgNb₂O₆ (MN) phase. An aqueous mixture of ammonium carbonate and ammonium hydroxide was used to precipitate Mg²⁺ and Nb⁵⁺ cations as carbonate and hydroxide respectively under basic conditions. This precipitate on heating at 750 °C produced MN powders. For comparison MN powders were prepared by the traditional solid state method. The phase content and the lattice parameters were studied by powder X-ray diffraction (XRD). Particle size and morphology of the particles were studied by scanning electron microscopy (SEM).     

Co-precipitation method for the preparation of ferroelectric CaBi<Subscript>4</Subscript>Ti<Subscript>4</Subscript>O<Subscript>15</Subscript>

A simple co-precipitation technique has been successfully applied for the preparation of pure single phase CaBi₄Ti₄O₁₅ (CBT) powders. Ammonium oxalate and ammonium hydroxide were used to precipitate Ca²⁺, Bi³⁺ and Ti⁴⁺ cations simultaneously. No pyrochlore phase was found while heating powder at 600 C and pure CBT phase was found to be formed by X-ray diffraction. Particle size and morphology was studied by transmission electron microscopy (TEM). The room temperature dielectric constant at 1 kHz is 400. The ferroelectric hysteresis loop parameters of these samples were also studied.     

A novel approach for synthesis of M-type hexaferrites nanopowders via the co-precipitation method

M-type hexaferrites; barium hexaferrite BaFe₁₂O₁₉ and strontium hexaferrite SrFe₁₂O₁₉ powders have been successfully prepared via the co-precipitation method using 5 M sodium carbonate solution as alkali. Effects of the molar ratio and the annealing temperature on the crystal structure, crystallite size, microstructure and the magnetic properties of the produced powders were systematically studied. The results indicated that a single phase of barium hexaferrite was obtained at Fe³⁺/Ba²⁺ molar ratio 12 annealed at 800–1,200 °C for 2 h whereas the orthorhombic barium iron oxide BaFe₂O₄ phase was formed as a impurity phase with barium M-type ferrite at Fe³⁺/Ba²⁺ molar ratio 8. On the other hand, a single phase of strontium hexaferrite was produced with the Fe³⁺/Sr²⁺ molar ratio to 12 at the different annealing temperatures from 800 to 1,200 °C for 2 h whereas the orthorhombic strontium iron oxide Sr₄Fe₆O₁₃ phase was formed as a secondary phase with SrFe₁₂O₁₉ phase at Fe³⁺/Sr²⁺ molar ratio of 9.23. The crystallite sizes of the produced nanopowders were increased with increasing the annealing temperature and the molar ratios. The microstructure of the produced single phase M-type ferrites powders displayed as a hexagonal-platelet like structure. A saturation magnetization (53.8 emu/g) was achieved for the pure barium hexaferrite phase formed at low temperature 800 °C for 2 h. On the other hand, a higher saturation magnetization value (M _s = 85.4 emu/g) was obtained for the strontium hexaferrite powders from the precipitated precursors synthesized at Fe³⁺/Sr²⁺ molar ratio 12 and thermally treated at 1,000 °C for 2 h.     

Gas sensing performance of nanocrystalline ZnO prepared by a simple route

The nanocrystalline powders of pure and Al³⁺-doped ZnO with hexagonal structure were prepared by a simple hydrothermal decomposition route. The structure and crystal phase of the powders were characterized by X-ray diffraction (XRD) and the microstructure by transmission electron microscopy (TEM). All the compositions exhibited a single phase, suggesting a formation of solid solution between Al₂O₃ and ZnO. DC electrical properties of the prepared nanoparticles were studied by DC conductivity measurements. The indirect heating structure sensors based on pure and doped ZnO as sensitive materials were fabricated on an alumna tube with Au electrodes. Gas-sensing properties of the sensor elements were measured as a function of concentration of dopant, operating temperature and concentrations of the test gases. The pure ZnO exhibited high response to NH₃ gas at an operating temperature of 200 °C. Doping of ZnO with Al³⁺ increased its response towards NH₃ and the Al³⁺-doped ZnO (3.0 wt% Al₂O₃) showed the maximum response at 175 °C. The selectivity of the sensor elements for NH₃ against different reducing gases like LPG, H₂S and H₂ was studied. The results on response and recovery time were also discussed.     

H2S sensing properties of La-doped nanocrystalline In2O3

The nanocrystalline powders of pure and La³⁺-doped In₂O₃ with cubic structure were prepared by a simple hydrothermal decomposition route. The structure and crystal phase of the powders were characterized by X-ray diffraction (XRD) and microstructure by transmission electron microscopy (TEM). All the compositions exhibited a single phase, suggesting a formation of solid solution in the concentration of doping investigated. Gas-sensing properties of the sensor elements were tested by mixing a gas in air at static state, as a function of concentration of dopant, operating temperature and concentrations of the test gases. The pure In₂O₃ exhibited high response towards H₂S gas at an operating temperature 150 °C. Doping of In₂O₃ with La³⁺ increases its response towards H₂S and La³⁺ (5.0 wt.% La₂O₃)-doped In₂O₃ showed the maximum response at 125 °C. The selectivity of the sensor elements for H₂S against different reducing gases was studied. The results on response and recovery time were also discussed.     

A co-precipitation technique to prepare BiTaO4 powders

A simple co-precipitation technique has been successfully used for the preparation of pure ultrafine single phase BiTaO₄. A standard ammonium hydroxide solution was used to precipitate Bi³⁺ and Ta⁵⁺ cations as hydroxides simultaneously under basic conditions. This precursor, on heating at 600 °C, produced product phase. This is the lowest temperature for the formation of BiTaO₄ phase so far reported in the literature. For comparison BiTaO₄ powders were also prepared by the traditional solid state method. The phase contents and lattice parameters were studied by the powder X-ray diffraction (XRD).     

A coprecipitation technique to prepare NaNbO3 and NaTaO3

A simple coprecipitation technique has been used successfully for the preparation of pure, ultrafine, single phases of NaNbO₃ (NN) and NaTaO₃ (NT). An alcoholic solution of ammonium carbonate and ammonium hydroxide was used to precipitate Na⁺ and Nb⁵⁺ (or Ta⁵⁺) cations under basic conditions as carbonate and hydroxide, respectively. On heating at 700°C, these precursors produce respective products. For comparison, both NN and NT powders were also prepared by the traditional solid state method. The phase purity and lattice parameters were studied by powder X-ray diffraction (XRD). The particle size and morphology were studied by scanning electron microscopy (SEM).     

Magnetic properties of nano-clusters lanthanum chromite powders doped with samarium and strontium ions synthesized via a novel combustion method

A novel approach to synthesize a single-phase orthorhombic perovskite lanthanum chromite LaCrO₃ clusters doped with Sm³⁺ and Sr²⁺ ions via gel combustion route was reported. The producing materials were synthesized using metal nitrates as oxidizers and triethanol amine (TEA), N-butyl amine (NBA) or ethylene diamine (EDA) as a fuel. The effect of the annealing temperature, type of organic fuel and the variation of the samarium and/or strontium substitution and its impact on crystal structure, crystallite size, microstructure and magnetic properties of the LaCrO₃ powders formed was systematically studied. The results revealed that a well crystalline single phase of pure LaCrO₃ can be achieved at annealing temperature from 800 to 1000 °C for 2 h. Moreover, each organic carrier materials exhibited a different degree of effectiveness in the synthesis of the mixed oxide powders. The crystal structure was influenced by doped Sm³⁺ and/or Sr²⁺ ions. The crystallite size of the produced powders was increased with the increase the annealing temperature, increasing the Sm³⁺ ion and the decrease of Sr²⁺ ion substitution. The microstructures of the produced powders were found to be nanoclusters octahedra-like shaped. The saturation magnetization of the LaCrO₃ powders increased continuously with an increase in the Sm³⁺ ion concentration and it decreased with an increase in the Sr²⁺ ion up to 0.3 at annealing temperature of 1000 °C for 2 h. The maximum saturation magnetization (0.279 emu/g) was achieved at the Sm³⁺ ion molar ratio 0.3 and annealing temperature 1000 °C. Moreover, wide coercivities can be obtained at different synthesis conditions (49.25 to 522  Oe).     

Phase transition of Er3+-doped Al2O3 powders prepared by the non-aqueous sol–gel method

The Er³⁺-doped Al₂O₃ powders have been prepared by the non-aqueous sol–gel method using the aluminum isopropoxide as precursor, acetylacetone as chelating agent, nitric acid as catalyzer, and hydrated erbium nitrate, as dopant under isopropanol environment. The phase structure and phase transition of the Er³⁺-doped Al₂O₃ powders were investigated by using thermogravimetry/differential thermal analysis (TG/DTA), Fourier transform infrared spectroscopy (FT-IR), and X-ray diffraction (XRD). The phase contents diagram for the Er-doped Al–O system with the doping concentration up to 5 mol% was described at the sintering temperature from 550 to 1250 °C. There were the three crystalline types of Er³⁺-doped Al₂O₃ phases, γ-, θ- and α-(Al, Er)₂O₃, and the two relative stoichiometric compounds composed of Al, Er, and O, ErAlO₃ and Al₁₀Er₆O₂₄ phases in the Er–Al–O phase contents diagram. The Er³⁺ doping suppressed crystallization of the γ and θ phases and delayed phase transition of the γ → θ and θ → α. The increased Er³⁺ doping concentration and the elevated sintering temperature enhanced the precipitation of the ErAlO₃ and Al₁₀Er₆O₂₄ phases. The preparation procedure for the Er³⁺-doped Al₂O₃ powders in the non-aqueous sol–gel process, including chelating, hydrolysis, peptization, doping and gelation, has a significant effect on the phase formation and its transition for the Er³⁺-doped Al₂O₃ powders.     

Magnetic and dielectric properties of polycrystalline La doped barium Z-type hexaferrite for hyper-frequency applications

La³⁺ ion substituted barium Z-type hexaferrite, Ba_3?XLa_XCo₂Fe₂₄O₄₁ powders (X = 0.0, 0.05, 0.10 and 0.15), have been synthesized using sol gel auto-combustion method. The phase identification, microstructure, complex permittivity, complex permeability and static magnetic properties of the samples were studied using X-ray diffraction, scanning electron microscopy, vector network analyzer and vibrating sample magnetometer. The results revealed that introducing La³⁺ ion instead of Ba²⁺ ion led to an obvious enhancement of the electromagnetic properties. The crystallite size of the produced powders was slightly increased with increasing La³⁺ content. The microstructure of the produced powders appeared as hexagonal-platelet like structure. As the La content increase, the static magnetic properties were increased, the real part of complex permittivity was increased while the imaginary part was decreased. Moreover, the real part of complex magnetic permeability was decreased and the imaginary part was increased. The reasons of the obtained results were discussed on basis of electromagnetic theory.     

Effect of fluorine impurity on the oxidation of silicon oxynitride ceramics doped with gadolinium oxide

Impurities in raw Si₃N₄ powders remain in intergranular glassy phases in Si₃N₄ and Si₂N₂O ceramics and degrade their high-temperature properties. Fluorine is one of the typical impurities in the raw powders. The oxidation rate of Si₂N₂O ceramics doped with Gd₂O₃ greatly varied with a difference in impurity contents (especially F) of the raw Si₃N₄ powders used. When a high concentration of impurity existed in the intergranular glassy phase, the rate of oxidation was controlled by O^2– diffusion through the glassy phase in the partly oxidized scale and unoxidized body; outward diffusion of Gd³⁺ occurred concurrently. On the other hand, when the impurity contents in the intergranular glassy phase was very low, the diffusion rate of ions (Gd³⁺, O^2–, etc.) in the glassy phase became very low (substantially zero in the oxidation at 1300°C). Only cristobalite (SiO₂) was formed on the surface. The rate of oxidation was controlled by O₂ diffusion through the cristobalite layer, and was very low.     

Effect of precursor morphology on the structural properties,optical absorption,and luminescence of BaI<Subscript>2</Subscript>:Eu<Superscript>2+</Superscript>,Eu<Superscript>3+</Superscript>

BaI₂:Eu²⁺,Eu³⁺ powders have been prepared by heat-treating BaCO₃:Eu³⁺ precursor powders of various morphologies in an iodinating agent atmosphere and their structural properties, morphology, optical absorption, and luminescence have been studied. The results demonstrate that the powders thus prepared consist of a mixture of crystalline hydrates of various compositions, dominated by BaI₂ ? 2Н₂О (sp. gr. C2/c), and that the Eu²⁺: Eu³⁺ ratio in the powders is determined by the morphology of the precursor.     

Luminescent impurity ion probe and low temperature phase of SrTiO3 nanoparticles

Abstract

Optical absorption and photoluminescence of Cr³⁺ impurity ion probe were studied on nanocrystalline SrTiO₃/Cr(0·1%) powders with an average particle size between 13 and 100 nm prepared by the Pechini type polymeric sol–gel method. Only O_h¹ cubic perovskite phase was revealed in the powders at room temperature. The optical absorption edge and the position of the zero-phonon emission R-line of the octahedral Cr³⁺ centres shifted to higher energies with decreasing particle size. The temperature shift of the R-line position appeared for all powders nearly the same as the 'dielectric related' one in the bulk crystals, evidencing that SrTiO₃ retains quantum paraelectric behaviour down to a particle size of about 10 nm. However, behaviour of the R-line position was unstable at low temperatures in powders with a particle size of about 13 and 20 nm. This instability was speculated as a manifestation of a possible low temperature ferroelectric phase transition in small enough SrTiO₃ nanoparticles.     

A crystallization conditions study of the Ti‐doped barium hexaferrite powders synthesized with the glass crystallization technique for microwave applications

For microwave applications Titanium doped M‐type hexagonal ferrites have been synthesized by means of glass crystallization technique varying the crystallization parameters and the melt doping concentrations. The chosen melt dopings were x = 5.4 and 7.2 mole‐% TiO₂ with the following basic composition (mole‐%): 40 BaO + 33 B₂O₃ + (27‐x) Fe₂O₃ + x TiO₂. We have studied the dependencies between the magnetic properties, the valence of the iron ions in the glasses and the powders, the formation of new dielectric phases and the microwave absorption. After the Ti⁴⁺ ions substitutions, the magnetocrystalline anisotropy changed, this effect was observed in the static magnetic properties (_JH_C and M_S) measured using a vibration sample magnetometer. Furthermore the Ti⁴⁺ ions preferably occupy mainly the 2a as well as slightly the 2b sites in the lattice of the barium hexaferrite, which are studied using Mössbauer spectroscopy. Besides, the X‐ray diffraction studies proved that the formation of the ferrimagnetic (BaFe_12‐xTi_xO₁₂) and dielectric (BaTi₆O₁₃) phases are dependent on the crystallization parameters. The controlled influencing of lattice sites occupation and of the Fe²⁺ content in the ferrimagnetic phase as well as the controlled formation of the dielectric phase rate during the annealing are possibilities to optimize the microwave absorption of Ti‐doped barium hexaferrite powders synthesized by glass crystallization technique.     

Preparation and studies of Eu and Tb co-doped Gd2O3 and Y2O3 sol-gel scintillating films

Eu³⁺ (2.5 at.%) and Tb³⁺ (0.005-0.01 at.%) co-doped gadolinium and yttrium oxide (Gd₂O₃ and Y₂O₃) powders and films have been prepared using the sol-gel process. High density and optical quality thin films were prepared with the dip-coating technique. Gadolinium (III) 2,4-pentadionate and yttrium (III) 2,4-pentadionate were used as precursors, and europium and terbium in their nitrate forms were used as doping agents. Chemical and structural analyses (infrared spectroscopy, X-ray diffraction and high-resolution transmission electron microscopy) were conducted on both sol-gel precursor powders and dip-coated films. The morphology of thin films heat-treated at 700 °C was studied by means of atomic force microscopy. It was shown that the highly dense and very smooth films had a root mean roughness (RMS) of 2 nm ± 0.2 (A = 0.0075 Tb³⁺) and 24 nm ± 3.0 (B = 0.01 Tb³⁺). After treatment at 700 °C, the crystallized films were in the cubic phase and presented a polycrystalline structure made up of randomly oriented crystallites with grain sizes varying from 20 to 60 nm. The X-ray induced emission spectra of Eu³⁺- and Tb³⁺-doped Gd₂O₃ and Y₂O₃ powders showed that Tb³⁺ contents of 0.005, 0.0075 and 0.01 at.% affected their optical properties. Lower Tb³⁺ concentrations (down to 0.005 at.%) in both systems enhanced the light yield.     

ZrB2-ceramic toughened by refractory metal Nb prepared by hot-pressing

ZrB₂–Nb (ZN) composites were prepared through hot-pressing at a temperature of 1800 °C. A contribution of Nb was believed a significant influence on the sinterability, microstructure and mechanical properties of ZN composites. The values of flexural strength of ZN composites rang from 395 to 773 MPa, who are dependent on Nb contents. The highest strength obtained for the ZN composite containing 25 vol.% Nb (773 MPa). A fracture toughness of 7.1 MPa m^1/2 of ZN was revealed, which was much higher than that of monolithic ZrB₂. The improvement in fracture toughness strongly depended on an introduction of Nb–ZrB₂ matrix. Crack deflection and branching were believed to be the toughening mechanism of ZN.     

Optical and magnetic properties of CuO/CuFe2O4 nanocomposites

CuO/c-CuFe₂O₄ nanocomposites have been synthesized via the oxalate precursor route. Effect of synthesis conditions on the crystal structure, microstructure, magnetic and optical properties of the formed powders was studied. The results indicated that pure CuO nanoparticles were obtained from the oxalate precursor annealed at 600 °C for 2 h. However, substitution of Cu²⁺ ion by Fe³⁺ ion (Cu_1?X Fe _X O, where X = 0, 0.05, 0.1 and 0.2) led to form of CuO/CuFe₂O₄ nanocomposites. The microstructures of the powders appeared as a monoclinic like shape. Furthermore, the band gap energy of the obtained CuO nanopowders was 1.41 eV and the value was slightly decreased by Fe³⁺ ion substitution. In addition, the formed CuO particles had weak ferromagnetic characteristics. However, the substitution Cu²⁺ ion by Fe³⁺ ion enhanced the magnetic properties of the formed composite as the result of increasing the CuFe₂O₄ phase formation. Hence, the saturation magnetization was increased from 0.13 to 9.8 emu/g by increasing the Fe³⁺ ion from 0 to 0.2.     

Hydrothermally-induced morphological transformation of GdVO4:Eu

The aim of the present study is to investigate the effect of a wide pH range on morphology and luminescence properties of europium-doped gadolinium vanadate (GdVO₄:Eu³⁺). GdVO₄:Eu³⁺ powders were hydrothermally synthesized at 180 °C for 24 h in a wide pH range. The as-synthesized powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence (PL) spectroscopy. The XRD results showed that GdVO₄:Eu³⁺ with the tetragonal structure formed at pH < 13 as a single phase and Gd(OH)₃ formed at pH ≥ 13 as a secondary phase. The SEM and TEM observations demonstrated the hydrothermally-induced morphological transformation of GdVO₄:Eu³⁺ powders by altering the pH of the synthesis solution. The possible mechanism for the morphological transformation was proposed. The intensities of the prominent peaks at 618 nm in the PL emission spectra of GdVO₄:Eu³⁺ powders considerably shift according to the specific morphology. The luminescence intensity of the octahedron- and rod-like GdVO₄:Eu³⁺ powders hydrothermally obtained at pH = 3 was the strongest one due to high packing density and high crystallinity as well as the extended reduction of the concentration of inherent defect states or adsorbed species.     

Photocatalytic activity of SnWO4 and SnW3O9 nanostructures prepared by a surfactant-assisted hydrothermal process

Flower-like α-SnWO₄ and rod-like SnW₃O₉ nanostructures in the form of a single and a mixed phase were prepared by hydrothermal process at 200 °C for 36 h in a wide pH range, with the assistance of dodecyltrimethylammonium bromide (DTAB). The effects of the pH of the synthesis solution and the Sn²⁺/W⁶⁺ molar ratio on phase compositions, structures and morphologies of the as-prepared powders were investigated. As a single phase, α-SnWO₄ and SnW₃O₉ could be hydrothermally prepared at pHs 7-8 and 1, respectively. In the form of a mixed phase, they could be hydrothermally obtained in the pH range of 2-6. The electron microscopy observations revealed that α-SnWO₄ powders presented flower-like structures with the diameter of 2-5 μm and SnW₃O₉ powders possessed rod-like structures with the width of 250 nm and the length of 2 μm. The X-ray photoelectron spectroscopy (XPS) results confirmed the reduction of tungsten from W⁶⁺ to W⁴⁺ during the hydrothermal process in accordance with the pH value. The UV-vis diffuse reflectance absorption spectra proved a strong visible-light absorbance of α-SnWO₄ powders in the range of 400-650 nm. Compared to phase-pure SnW₃O₉ and the mixture of α-SnWO₄ and SnW₃O₉ phases, phase-pure α-SnWO₄ exhibited a good photocatalytic activity for the degradation of methyl orange under visible-light irradiation. After several cycles, the α-SnWO₄ sample retains high degradation efficiency.     

Luminescence properties of BaTiO3:Eu obtained via microwave stimulated hydrothermal method

BaTiO₃ nanocrystalline powders doped with the Eu³⁺ ions have been prepared using microwave stimulated hydrothermal method (MSHM). Structure, average grain size and morphology of the BaTiO₃:Eu³⁺ were analyzed by means of the X-ray powder diffraction measurements, Raman spectroscopy and SEM microscopy. The luminescence properties and decay times of the hydrothermal BT:Eu³⁺ nanocrystalline powders have been investigated as a function of the grain size, dopant concentration, preparation conditions and sintering temperature. It was found that the studied properties are strongly dependent on the grain size of BaTiO₃:Eu³⁺ nano-crystallites.