Study on synthesis and evolution of sodium potassium niobate ceramic powders by an oxalic acid-based sol–gel method

Potassium sodium niobate (KNN) ceramic powders by a variation of sol–gel method is synthesized. The metal precursors used for the KNN synthesis are potassium carbonate, sodium carbonate and niobium hydroxide, ethylene glycol are used as chelating and esterification agent, respectively. The effects of amount of oxalic acid (OA) and ethylene glycol (EG), pH value on the stability of the precursor sol were investigated. The evolution of (K_0.5Na_0.5)NbO₃ crystal phase was also investigated by XRD, IR, SEM and TG-DTA. The results showed that stable precursor sol was formed when n_(OA):n_(Mn+) = 3:1, n_(OA):n_(EG) = 1:2 and pH value was in the range of 2.5–3.5. Xerogel was sintered in the range of 500–650 °C to prepare K₆Nb_10.88O₃₀ and Na₂Nb₄O₁₁ powder. Then the compound was sintered at 750 °C to produce perovskite (K_0.5Na_0.5)NbO₃ ceramic powders. The grain size is about 100–200 nm.     

Structural and morphological characteristics of (Pb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)TiO<Subscript>3</Subscript> powders obtained by polymeric precursor method

Pb_1−x Sr _x )TiO₃ powders with different compositions (x = 0, 0.10, 0.50, 0.90 and 1) were synthesized by the polymeric precursor method and heat treated at 800 °C for 2 h under air atmosphere. The thermogravimetric and differential scanning calorimetry analyses were performed in the range from 25 to 800 °C in order to estimate the stages corresponding to the water evaporation, organic decomposition and crystallization of these materials. X-ray diffraction patterns and Rietveld analyses showed that the (Pb_1−x Sr _x )TiO₃ phases with strontium content up to x = 0.1 crystallize in a tetragonal structure. The micrographs obtained by scanning electron microscopy and transmission electron microscopy showed that the powders have agglomerated nature, presenting irregular morphologies and polydisperse particle size distribution. The energy dispersive X-ray spectrometry indicated the presence of pure (Pb_0.50Sr_0.50)TiO₃ phase.     

Metamagnetic DyAlO<Subscript>3</Subscript> nanoparticles with very low magnetic moment

Nanocrystalline dysprosium monoaluminate (DyAlO₃) has been synthesized by modified sol–gel method after sintering the precursor gel at 950 °C. The micro-structural features have been verified by X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy. The XRD pattern confirms the formation of single-phase DyAlO₃; the average size of the nanoparticles is 50 nm. X-Ray photoelectron spectroscopy has been used to study the chemical composition and bonding in the samples. The binding energies of core-level electrons in Dy, Al and O in DyAlO₃ nanopowder have been found slightly shifted compared to the respective values of the same elements. Both AC and DC magnetic susceptibilities have been measured in the temperature range 2–300 K. Unusually low effective magnetic moment of Dy³⁺, μ_eff = 0.38, has been derived from the inverse magnetic susceptibility–temperature plot between 4 and 252 K. The Nèel temperature, T_N = 3.920 K and exchange interaction constant J/k = −1.74 K, have been also determined.     

Synthesis of Y<Subscript>3−x</Subscript>Lu<Subscript>x</Subscript>Al<Subscript>3</Subscript>MgSiO<Subscript>12</Subscript> garnet powders by sol–gel method

Y_3−xLu_xAl₃MgSiO₁₂ (x = 0–3) garnet powders were synthesized by an aqueous sol–gel method based on metal chelates with 1,2-ethanediol in aqueous media. Target samples were characterized by powder X-ray diffraction (XRD), scanning electron microscopy and reflection spectra. XRD analysis revealed that sintering of polycrystalline Y_3−xLu_xAl₃MgSiO₁₂ powders at 1,600 °C results in single-phase garnet materials.     

Mesoporous and nanocrystalline sol–gel derived NiTiO3 at the low temperature: Controlling the structure,size and surface area by Ni:Ti molar ratio

Nanocrystalline nickel titanate (NiTiO₃) thin films and powders with mesoporous structure were produced at the low temperature of 500 °C by a straightforward particulate sol–gel route. The sols were prepared in various Ni:Ti molar ratios. X-ray diffraction and Fourier transform infrared spectroscopy revealed that the powders contained mixtures of the NiTiO₃ and NiO phases, as well as the anatase-TiO₂ and the rutile-TiO₂ depending on the annealing temperature and Ni:Ti molar ratio. Moreover, it was found that Ni:Ti molar ratio influences the preferable orientation growth of the nickel titanate, being on (202) planes for the nickel dominant powders (Ni:Ti ≥ 75:25) and on (104) planes for the rest of the powders (Ni:Ti: ≤ 50:50). The average crystallite size of the powders annealed at 500 °C was in the range 1.5–2.4 nm and a gradual increase occurred up to 8 nm by heat treatment at 800 °C. The activation energy of crystal growth decreased with an increase of Ni:Ti molar ratio, calculated in the range 24.93–37.17 kJ/mol. Field emission scanning electron microscope analysis revealed that the deposited thin films had mesoporous and nanocrystalline structure with the average grain size of 20–35 nm. Moreover, atomic force microscope images presented that the thin films had a hill-valley like morphology with roughness mean square in the range 41–57 nm. Based on Brunauer–Emmett–Taylor analysis, the synthesized powders showed mesoporous structure containing pores with needle and plate like shapes. The mesoporous structure of the powders was stable at high annealing temperatures and one of the highest surface areas (i.e., 156 m²/g) reported in the literature was obtained for the powder containing Ni:Ti = 50:50 at 500 °C.     

Preparation of LiTaO<Subscript>3</Subscript> nanoparticles by a sol–gel route

A sol–gel route was developed to prepare pure ultrafine LiTaO₃ powders using Ta₂O₅, Li₂CO₃, citric acid (CA) as chelating agent, ethylene glycol (EG) as esterification agent and polyethylene glycol (PEG) as dispersant. The effects of pH value and heat treatment temperature of powder precursor on the synthesis of LiTaO₃ powders were investigated. The phase content and morphology of the final product were evaluated by XRD, SEM and TEM. A transparent gel was produced when heating a mixed-solution of CA, EG, Li and Ta ions with a molar ratio of [CA]:[EG]:[Li]:[Ta] = 3.0:6.0:1.0:1.0 and 2‰ PEG additions with a pH value of 7 at water bath temperature of 80 °C. The results showed that single phase LiTaO₃ powders with average particle sizes of nanometers were produced after heat treatment of the powder precursor at 650, 700, 800, and 900 °C respectively for 2 h.     

BaTi<Subscript>1–x</Subscript>Zr<Subscript>x</Subscript>O<Subscript>3</Subscript> nanopowders prepared by the modified Pechini method

The results reported here based on a study of BaTi_1–xZr_xO₃ (x=0, 0.2 and 1) nanometric powders prepared by the modified Pechini method. The powder samples annealed from 600 to 1000°C/2 h were characterized by thermogravimetric analysis (TG), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The decomposition reactions of resins were studied using thermal analysis measurements. The barium titanate zirconate system presented just one orthorhombic phase. Furthermore, this study produced BaTiO₃ powders with a tetragonal structure using shorter heat treatments and less expensive precursor materials than those required by the traditional methods.     

Eu,Dy co-doped SrAl<Subscript>2</Subscript>O<Subscript>4</Subscript> phosphors prepared by sol–gel-combustion processing

SrAl₂O₄:Eu²⁺, Dy³⁺ powders were synthesized by sol–gel–combustion process using metal nitrates as the source of metal ions and citric acid as a chelating agent of metal ions. The amounts of citric acid in mole were two times those of the metal ions. By tracing the formation process of the sol–gel, it is found that decreasing the amount of NO₃ ⁻ in the solution is necessary for the formation of transparent sol and gel, and the dropping of ethanol into the precursor solution can decrease the amount of NO₃ ⁻ in the solution. By combusting citrate sol at 600 °C, followed by heating the resultant combustion ash at 1,100–1,300 °C in a weak reductive atmosphere containing active carbon, SrAl₂O₄:Eu²⁺, Dy³⁺ phosphors can prepared. X-ray diffraction, Thermogravimetry–differential thermal analysis, scanning electron microscopy and fluorescence spectrophotometer were used to investigate the formation process and luminescent properties of the as-synthesized SrAl₂O₄:Eu²⁺, Dy³⁺. The results reveal that the SrAl₂O₄ crystallizes completely when the combustion ash was sintered at 1,200–1,300 °C. The excitation and emission spectra indicate that excitation broadband mainly lies in a visible range and the phosphors emit strong light at 510 nm under the excitation of 348 nm. The afterglow of phosphors lasts for over 10 h when the excited source is cut off.     

A facile synthesis of NaNbO<Subscript>3</Subscript> powders by decomposition of ammonium niobium oxalate with sodium salt at low temperature

A facile synthesis of NaNbO₃ powders was performed by solid-state reaction at low temperature. Stoichiometric ammonium niobium oxalate and Na₂CO₃ were mixed in water and then calcined at different temperatures for various times after drying. A combination of X-ray diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared (FTIR) spectroscopy and thermogravimetric (TG) analysis was used to characterize the product and precursor compound. The XRD patterns show that single-phase NaNbO₃ powders with high crystallinity can be synthesized at 425 °C for 15 min. The particle size from XRD data is found to be about 40 nm for NaNbO₃ powders calcined at 500 °C for 3 h, which is in good agreement with SEM data. The SEM photograph shows that NaNbO₃ powders are cuboid-like and well crystallized when calcination at 800 °C for 3 h. The product compositions prepared using other sodium reactants, such as HCOONa and NaNO₃, are also discussed.     

High surface area sol–gel alumina–titania nanocatalyst

Alumina–titania mixed oxide nanocatalysts with molar ratios = 1:0.5, 1:1, 1:2, 1:5 have been synthesized by adopting a hybrid sol–gel route using boehmite sol as the precursor for alumina and titanium isopropoxide as the precursor for titania. The thermal properties, XRD phase analysis, specific surface area, adsorption isotherms and pore size details along with temperature programmed desorption of ammonia are presented. A specific surface area as high as 291 m²/g is observed for 1:5 Al₂O₃/TiO₂ composition calcined at 400 °C, but the same composition when calcined at 1,000 °C, resulted in a surface area of 4 m²/g, while 1:0.5 composition shows a specific surface area of 41 m²/g at 1,000 °C. Temperature programmed desorption (of ammonia) results show more acidic nature for the titania rich mixed oxide compositions. Transmission electron microscopy of low and high titania content samples calcined at 400 °C, shows homogeneous distribution of phases in the nano range. In the mixed oxide, the particle size ranges between 10–20 nm depending on titania content. The detailed porosity data analysis contributes very much in designing alumina–titania mixed oxide nanocatalysts.     

Rapid synthesis of DyFeO<Subscript>3</Subscript> nanopowders by auto-combustion of carboxylate-based gels

EDTA and citric acid as two typical chelating reagents with multi-carboxyl groups were used to prepare DyFeO₃ nanopowders, respectively. The experimental results show that all of the carboxylate-based gels exhibited auto-propagating combustion behaviors. The XRD results indicate that DyFeO₃ single phase can be formed directly with CA/MN (citric acid to metal nitrate mole ratio) = 1 when the calcination temperature was above 700 °C. The specimen with EA/MN (EDTA to metal nitrate mole ratio) = 1 had the minimum crystallite size of 33 nm. The SEM images show that the as-burnt powders prepared with EDTA had more excellent dispersibility feature and clearer grain boundaries than that of citric acid. The magnetic measurement results show that DyFeO₃ nanopowders displayed antiferromagnetism characteristics at low temperatures due to the strong exchange interaction between Fe sublattice. As the ambient temperature increased, there was a transition from antiferromagnetism to paramagnetism in DyFeO₃ nanopowders.     

Synthesis of terbium doped calcium phosphate nanocrystalline powders by citric acid sol–gel combustion method

Terbium doped calcium phosphate (Tb-doped CaP) nanocrystalline powders were synthesized by the citric acid sol–gel combustion method. The phase composition, morphology and luminescent property of Tb-doped CaP nanocrystalline powders were characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, fluorescence spectrophotometer and fluorescence microscopy. At 700 °C, Tb-doped CaP nanocrystalline powders are composed of HAP (main phase) and β-TCP (minor phase) with Tb doping content of 0.5–4%. SEM and TEM observations show that the 4% Tb-doped CaP nanocrystalline powders are about 50–150 nm spherical particles. The 4% Tb-doped CaP nanocrystalline powders exhibit the strongest emission at 548 nm (λ_excitation = 240 nm) and show strong green fluorescence under fluorescence microscopy.     

Effect of the nature of anions of aluminum salts used to synthesize a precursor of the Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-ZrO<Subscript>2</Subscript> ceramics on the stabilization of the tetragonal modification of zirconium dioxide

Samples of a precursor for an aluminum oxide ceramics reinforced with zirconium oxide were synthesized by hydrolysis of various aluminum salts in the presence of a ZrO₂ sol under conditions of urea decomposition at 90°C and pH < 4 maintained, with hydrolysis products deposited onto the surface of ZrO₂ sol particles. It was found that the nature of a salt anion affects the interaction of hydrolysis products of the aluminum cation with the surface of ZrO₂ sol particles. The structure of products formed in thermal treatment of samples of a precursor for Al₂O₃-ZrO₂ (T = 1250°C) was characterized by X-ray phase analysis and scanning electron microscopy. The phase transition temperatures of the oxides Al₂O₃ and ZrO₂ contained in the precursor were estimated using the results of thermal analysis of the samples in the temperature range 20–1300°C.     

Synthesis and characterization of novel InVO<Subscript>4</Subscript>–TiO<Subscript>2</Subscript> thin films using the low temperature sol

The InVO₄ sol was obtained by a mild hydrothermal treatment (the precursor precipitation solution at 423 K, for 4 h). Novel visible-light activated photocatalytic InVO₄–TiO₂ thin films were synthesized through a sol–gel dipping method from the composite sol, which was obtained by mixing the low temperature InVO₄ sol and TiO₂ sol. The photocatalytic activities of the new InVO₄–TiO₂ thin films under visible light irradiation were investigated by the photocatalytic discoloration of methyl orange aqueous solution. The thin films were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and UV–Vis absorption spectroscopy (UV–Vis). The results revealed that the InVO₄ doped thin films enhanced the methyl orange degradation rate under visible light irradiation, 3.0 wt% InVO₄–TiO₂ thin films reaching 80.1% after irradiated for 15 h.     

Preparation of Bi0.4Sb1.6Se3xTe3(1−x) hexagonal rods and effect of Se on structure and electrical property

In the current study, novel hexagonal rods based on Bi_0.4Sb_1.6Te₃ raw materials and dispersed with x amounts of Se (x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) in the form Bi_0.4Sb_1.6Se_3xTe_3(1−x) were synthesized via a standard solid-state microwave route. The morphologies of these rods were explored using field emission scanning electron microscopy (FESEM). The crystalline of the powders were examined by X-ray diffraction (XRD), which showed that the powders of 0.0 ≤ x ≤ 0.8 samples can be indexed as the rhombohedral phase, whereas the sample with x = 1.0 has an orthorhombic phase structure. The influence of variations in Se content on thermoelectric properties was studied in the temperature range of 300–523 K. The alloying of Se in Bi_0.4Sb_1.6Te₃ effectively caused a decrease in hole concentration and, thus, a decrease in electrical conductivity and an increase in Seebeck coefficient. The maximal power factor measured in the present work was 7.47 mW/m K² at 373 K for the x = 0.8 sample.     

Low temperature synthesis of highly ion conductive Li7La3Zr2O12–Li3BO3 composites

Highly lithium ion conductive composites with Al-doped Li₇La₃Zr₂O₁₂ (LLZ) and amorphous Li₃BO₃ were prepared from sol–gel derived precursor powders of LLZ and Li₃BO₃. Precursor LLZ powders with cubic phase were obtained by a heat treatment of the precursor dried gel at 600 °C. Pellets of the mixture of the obtained LLZ and Li₃BO₃ were first held at 700 °C, and then successively sintered at 900 °C. Density of the sintered pellet with Li₃BO₃ was larger than that of the pellet without Li₃BO₃. From the TEM observation, the pellets were found to consist of cubic LLZ and amorphous Li₃BO₃. Total electrical conductivity of the obtained LLZ–Li₃BO₃ composite was 1 × 10^− 4 Scm^− 1 at 30 °C.     

Preparation of superhydrophobic PET fabric from Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> hybrid: geometrical approach to create high contact angle surface from low contact angle materials

In this work we prepare high contact Poly Ethylene Terephthalate (PET) fabric surface from low contact angle materials. Superhydrophobic PET fabric is prepared by coating the fabric with hybrid Al₂O₃–SiO₂ sol. In this case, the high contact angle Al₂O₃–SiO₂ hybrid is created from low contact angle Al₂O₃ and SiO₂ precursors. PET treated with hybrid Al₂O₃–SiO₂ exhibit Water Contact Angle (WCA) as 150°, while PET treated with individual Al₂O₃ sol or SiO₂ sol exhibits lower WCA, (Al₂O₃ WCA = 137°; SiO₂ WCA = 141°). FESEM and AFM investigations show that the hybrid Al₂O₃–SiO₂ sol and individual Al₂O₃ or SiO₂ sol imparted different roughness geometry on the PET fabric surface. We observe surface structure of fish fin-like, particle-like and hybrid fin-particle for treated PET fabric with; Al₂O₃, SiO₂ and hybrid Al₂O₃–SiO₂ sol, under FESEM and AFM observations.AFM observations show the evolution of roughness (Ra) dimension of different surface structures with the order of: SiO₂ < Al₂O₃ < Al₂O₃–SiO₂ (Ra = 31, 63 and 273 nm). We believe that the disparity of the surface geometries lead into different surface WCA. FTIR spectra of Hybrid Al₂O₃–SiO₂ shows additional peak at 902, 850, 557, and 408 cm⁻¹ which can be ascribed to the hybridization structure.     

Synthesis and structural properties of (Y,Sr)(Ti,Fe, Nb)O3−δ perovskite nanoparticles fabricated by modified polymer precursor method

The yttrium, iron and niobium doped-SrTiO₃ powders have been successfully fabricated by a modified low–temperature synthesis method from a polymer complex. The usage of strontium hydroxide precursor instead of conventional strontium nitrate or strontium carbonate provides to the possibility of significant decrease of annealing temperature. It allows to prepare a material with sphere-shape grains of nanometric size (15–70 nm). The results of thermal analysis indicate that the crystallization of precursor takes place at different stages. The product after heat treatment at 600 °C for 3 h in air was also characterized by X-Ray diffraction method (XRD) and Fourier transform – infrared spectroscopy (FT-IR). After the crystallization and the impurity removal process, a single-phase material was obtained in case of all analyzed samples. The morphology of obtained nano-powders was also studied by a scanning electron microscopy (SEM). It can be concluded, that this method allows obtaining a perovskite phase of a metal doped SrTiO₃ with nanometric particles.     

Sol–gel synthesis and crystallization behaviour of β-spodumene

Lithium aluminum silicate powders in the form of β-spodumene were synthesized through sol–gel technique by mixing boehmite sol, silica sol and lithium salt. The gel and oxide powders were characterized by thermogravimetry, differential thermal analysis (DTA), X-ray diffraction (XRD), Fourier transformed infrared (FTIR) spectroscopy and scanning electron microscopy. DTA, XRD and FTIR results confirmed that crystallization of β-spodumene took place at about 800 °C. The tiny crystallites with average size less than 1 μm appeared when the gel powders were sintered at 800 °C. A substantial increase of the crystal grain size was observed with increasing sintering temperatures.     

Structural and optical characterizations of rare earth pentaphosphates LnP5O14 (Ln = La, Gd) synthesized by the sol–gel process

The sol–gel chemistry route has successfully been used to prepare samples of LnP₅O₁₄ (Ln = La, Gd) pentaphosphates from lanthanides chlorides and phosphorous pentoxide dissolved in isopropanol. Crystallized powders of single phase were obtained after calcination of gels at 350 °C. The structural characterizations of materials were investigated by means of X-ray diffraction as well as infrared and Raman spectroscopies, whereas their thermal behavior has been studied by differential thermal analyse and thermogravimetric analyses. Powders morphology was analyzed by means of scanning electron microscopy and laser granulometry. The photoluminescence properties of the Eu³⁺ ions in sol–gel derived LaP₅O₁₄ and GdP₅O₁₄ samples were investigated and compared with homologous samples synthesized by the conventional solid state reaction.