Synthesis of pure phase BiFeO3 powders in molten alkali metal nitrates

Pure phase BiFeO₃ powders were successfully synthesized in molten alkali metal nitrates (KNO₃–NaNO₃) at 500 °C. The as-prepared BiFeO₃ had a rhombohedral structure which was studied using X-ray diffraction. The plate-like morphologies were investigated through scanning electron microscopy and transmission electron microscopy. The average length and width of BiFeO₃ plates were 400 and 200 nm, respectively. Furthermore, the mechanism of formation of BiFeO₃ was also discussed through X-ray diffraction, thermogravimetry, differential thermal analysis and mass spectrometry.     

Modification of grain boundary structure of SrTiO3 using hydroxyl additives

Strontium titanate (SrTiO₃) is a typical perovskite material due to its crucial properties such as a good dielectric constant and favorable conductivity. Early studies reported that hydroxyl (OH) groups can manipulate the shape of the SrTiO₃ surface and control the crystal growth by changing the surface energy. This concept may be used to increase the population of (111) surfaces to form more Σ3 grain boundaries, which have lower energy for high ionic conductivity. In this study, two common OH additives, glycerol and 1,2-propanediol, were used for preparation of SrTiO₃ powders. SrTiO₃ powders were synthesized by spray pyrolysis and then sintered to form polycrystalline SrTiO₃. The results showed that the additive-treated SrTiO₃ powders had a higher population of (111) planes and exhibited higher conductivity than did un-treated powder. In addition, the highest population of Σ3 grain boundaries was generated by 1,2-propanediol treatment (11.0 ± 0.8%) of SrTiO₃, followed by glycerol treatment (10.0 ± 0.3%) and no treatment (2.5 ± 0.2%), respectively.     

Synthesis of CuGa2O4 nanoparticles by precursor and self-propagating combustion methods

Copper gallate spinels, CuGa₂O₄, have been synthesized by two wet chemical routes: precursor method and self-propagating combustion involving a glycine-nitrate system. All complex precursors have been characterized by chemical analysis, infrared spectroscopy (IR), ultraviolet visible spectroscopy (UV–vis), electron paramagnetic resonance spectroscopy (EPR), thermal analysis and scanning electron microscopy (SEM). The copper gallate spinel oxides have been further investigated by X-ray diffraction (XRD), SEM, IR, UV–vis, magnetic measurements and EPR. The crystallite size of the copper gallate was found about 280 Å.     

Fabrication of Yb-doped Lu2O3-Y2O3-La2O3 solid solutions transparent ceramics by self-propagating high-temperature synthesis and vacuum sintering

A comparative analysis of sintering and grain growth processes of lutetium oxide and lutetium-yttrium-lanthanum oxides solid solutions, as well as optical properties, luminescence and laser generation of (Lu_xY_0.9-xLa_0.05Yb_0.05)₂O₃ transparent ceramics are reported. Fabrication of highly dispersed initial powders of these compounds was performed via nitrate-glycine self-propagating high-temperature synthesis (SHS) method. The powders were compacted at 300?MPa and vacuum sintered at temperatures up to 1750?°С. Optical ceramics of (Lu_0.65Y_0.25La_0.05Yb_0.05)₂O₃ elemental composition were shown to have the highest in-line transmittance, which achieved 78% at the wavelength of 800?nm. Generation of laser radiation at a wavelength of 1032?nm with the differential efficiency of 20% was demonstrated in the (Lu_0.65Y_0.25La_0.05Yb_0.05)₂O₃ ceramics.     

Preparation and microstructural characterization of (Nd1−xGdx)2(Ce1−xZrx)2O7 solid solutions

(Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ (0 ≤ x ≤ 1.0) powders with an average particle size of 100 nm were synthesized with chemical-coprecipitation and calcination method, and were characterized by X-ray diffractometry and scanning electron microscopy. The sintering behaviour of (Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ powders was studied by pressureless sintering at 1600–1700 °C for 10 h in air. The relative densities of (Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ solid solutions increase with increasing the sintering temperature, and gradually decrease with increasing the content of neodymium and cerium at identical temperature levels. (Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ solid solutions have a single phase of defect fluorite-type structure among all the composition combinations studied. The lattice parameters of (Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ solid solutions agree well with the Vegard's rule.     

Synthesis and characterization of multiferroic BiFeO3 powders fabricated by hydrothermal method

The influence of processing parameters on phase formation and particle size of hydrothermally synthesized BiFeO₃ powders was investigated. BiFeO₃ powder was synthesized by dissolving bismuth nitrate and iron nitrate in KOH solution at temperatures ranging from 150 to 225 °C. X-ray diffraction patterns and scanning electron microscopy observation indicated that rod-like α-Bi₂O₃ phase was formed at initial stage of reaction and dissolved into ions to form thermodynamically stable BiFeO₃ phase. Single-phase perovskite BiFeO₃ has been formed using a KOH concentration of 8 M at a temperature of ≥175 °C in a 6 h reaction period. BiFeO₃ particle growth was promoted by lowering the KOH concentration, or increasing the duration time or reaction temperature. The effects of processing conditions on the formation of crystalline BiFeO₃ powders were discussed in terms of a dissolution–precipitation mechanism. The magnetization of the BiFeO₃ powders at room temperature showed a weak a ferromagnetic nature.     

Synthesis and photoluminescence properties of La1−xAlO3:xTb green phosphors for white LEDs

Tb³⁺-doped La_1−xAlO₃ phosphor powders are successfully synthesized by the solution combustion method, using citric acid as the combustion fuel. The crystal structure and photoluminescence properties of La_1−xAlO₃:xTb³⁺ phosphors are studied, depending on Tb³⁺ content. The strongest emission peak is found at 543 nm, which originates from the ⁵D₄→⁷F₅ transition of Tb³⁺ ions, indicating green emission. Among the fabricated phosphors, the La_0.9AlO₃:0.1Tb³⁺ phosphor emits the strongest green light. The excellent luminescent properties make it a possible candidate for white light-emitting diodes and various photonic applications.     

The luminescence spectroscopy and thermal stability of red-emitting phosphor Ca9Eu(VO4)7

Eu-based vanadate Ca₉Eu(VO₄)₇ phosphor was synthesized by the solid state reaction method and was characterized by X-ray powder diffraction (XRD). The photoluminescence excitation and emission spectra, fluorescence decay curves and the dependence of luminescence intensity on temperature were investigated. The phosphor can be efficiently excited by near UV light to realize an intense red luminescence (614 nm) corresponding to the electric dipole transition ⁵D₀ → ⁷F₂ of Eu³⁺ ions. The crystallographic site-occupations of the Eu³⁺ ions in Ca₉Eu(VO₄)₇ were investigated by the site-selective excitation and emission spectra, and the fluorescence decay curves in the ⁵D₀ → ⁷F₀ region using a pulsed, tunable, narrowband dye laser. The red luminescence together with the thermal stability was discussed on the base of the Eu³⁺ site-distribution in Ca₉Eu(VO₄)₇ host.     

Novel hydroxide precursors to prepare NaNbO3 and KNbO3

Firstly, fresh niobium hydroxide was precipitated from NbF₅ solution using an aqueous ammonium hydroxide under basic conditions. Then a simple procedure of mixing potassium (or sodium) and niobium hydroxides together and heating at a low temperature (<400 °C) produced KNbO₃ (KN) or NaNbO₃ (NN) powders. This is the lowest temperature so far reported for the formation of these phases. The phase content and lattice parameters are determined by X-ray diffraction (XRD). The average particle size and morphology were studied by scanning electron microscopy (SEM).     

One-pot hydrothermal synthesis of MoS2 nanosheets/C hybrid microspheres

Uniform MoS₂ nanosheets/C hybrid microspheres with mean diameter of 320 nm have been successfully synthesized via a facile one-pot hydrothermal route by sodium molybdate reacting with sulfocarbamide in d-glucose solutions. The products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). XRD patterns showed that the MoS₂ was kept as a two-dimensional nanosheet crystal and C was retained as amorphous even after their annealing treatment at 800 °C. TEM and SEM images indicated that the MoS₂ nanosheets were uniformly dispersed in the amorphous carbon. The experiment results also revealed that the appropriate amount of d-glucose had an obvious effect on the formation of uniform MoS₂ nanosheets/C hybrid microspheres. A possible formation process of MoS₂ nanosheets/C hybrid microspheres was preliminarily presented.     

Effects of phase and chemical composition of precursor on structural and morphological properties of (Lu0.95Eu0.05)2O3 nanopowders

Europium-doped lutetium oxide nanopowders have been synthesized by the co-precipitation technique using ammonium hydrogen carbonate as a precipitant. Effects of chemical and phase composition of carbonate precursors on the morphology and sinterability of (Lu_0.95Eu_0.05)₂O₃ nanopowders have been studied. Two types of precursors have been obtained differing by the molar ratio R=NH₄HCO₃/Lu³⁺: a mixture of crystalline Lu_0.95Eu_0.05(OH)(CO₃)·4H₂O and unidentified amorphous phases at R=4–7 and crystalline Lu_0.95Eu_0.05(H₂O)_x(HCO₃)₃·nH₂O precursor at R=8–20. The two-phase precursor consists of spherulite-like aggregates, while the crystalline one is characterized by plate-like morphology. Calcination of Lu_0.95Eu_0.05(H₂O)_x(HCO₃)₃·nH₂O leads to formation of (Lu_0.95Eu_0.05)₂O₃ nanopowders that inherit the precursor morphology, while no morphology succession is observed for (Lu_0.95Eu_0.05)₂O₃ nanopowders obtained by heat treatment of the two-phase precursor. Calcination of the two-phase mixture leads to breakdown of the spherulites and to formation of equiaxed particles with an average diameter of 40 nm with the standard deviation of particle size distribution of about 15%. The obtained low-agglomerated nanopowders were used in vacuum sintering to produce (Lu_0.95Eu_0.05)₂O₃ optical ceramics with in-line transmittance of 41% at 611 nm.     

Processing of Bi1.5ZnNb1.5O7 ceramics for LTCC applications: Comparison of synthesis and sintering methods

Bismuth zinc niobate posses a cubic pyrochlore structure and normally is obtained by the conventional solid-state reaction. The great disadvantage of this method is the lack of chemical homogeneity, requiring high synthesis and sintering temperatures (higher than 1000 °C), which is an impeditive for BZN application in LTCC with silver as the internal electrode. The aim of this paper is to compare, from synthesis to sintering, BZN ceramics, derived either from chemically or conventionally synthesized powders, sintered either in both conventional oven for 2 h or microwave oven for 15 min. The results showed that chemically synthesized BZN ceramics sintered in microwave oven at 900 °C for 15 min presented a relative density of 97%, while those obtained by conventional method required 1000 °C to reach the same density. Despite the short period for thermal treatment in microwave oven, the electrical properties of BZN ceramics are compatible with those sintered in conventional oven for 2 h.     

Nanocrystalline BaAl2O4 powders prepared by aqueous combustion synthesis

In order to achieve BaAl₂O₄ formation via combustion synthesis, two types of recipes were designed: single fuel recipes (urea, glycine, β-alanine or hexamethylenetetramine) and innovative fuel mixture recipes (urea+glycine, urea+β-alanine and urea+hexamethylenetetramine respectively). No combustion reactions were noticed in the case of single fuel recipes based on urea or hexamethylenetetramine. The only crystalline phase present in the case of the powders obtained in such a way was unreacted Ba(NO₃)₂. Glycine and β-alanine generated smoldering reactions, leading to the formation of black powders, which consist of Ba(NO₃)₂, BaCO₃ and traces of BaAl₂O₄ (in the case of β-alanine). Fuel mixture recipes proved to be better than single fuel recipes, yielding BaAl₂O₄ as main crystalline phase. The specific surface area of the resulted powders ranges between 2.0 and 3.9 m²/g. The urea and glycine fuel mixture was the most efficient yielding BaAl₂O₄ with an average crystallite size of 60 nm.     

Comparison of the microwave-induced combustion and solid-state reaction for the synthesis of LiMn2O4 powder and their electrochemical properties

In the current research, we proposed a new method called microwave-induced combustion synthesis to produce LiMn₂O₄ powders. The microwave-induced combustion synthesis entails the dissolution of metal nitrates, and urea in water, and then heating the resulting solution in a microwave oven. Spinel LiMn₂O₄ powders were successfully synthesized by microwave-induced combustion. The microwave-heated LiMn₂O₄ powders annealed at various temperatures in the range of 600–800 °C were determined. The resultant powders were characterized by X-ray diffractometer (XRD), and scanning electron microscopy (SEM). The annealed samples were used as cathode materials for lithium-ion battery, for which their discharge capacity and electrochemical characteristic properties in terms of cycle performance were also investigated. The LiMn₂O₄ cell provides a high initial capacity of 133 mAh/g and excellent reversibility. The excellent capacity and reversibility were attributed to LiMn₂O₄ powders with small and uniform particle size produced by microwave-induced combustion synthesis.     

Effect of Na2CO3 on hexagonal boron nitride prepared from urea and boric acid

Formation of hexagonal boron nitride (hBN) from a precursor obtained by the reaction of urea and boric acid was studied in nitrogen, ammonia and argon atmospheres in 700-1200 °C temperature range. Effect of sodium carbonate (Na₂CO₃) addition on this process was investigated. Reaction products were subjected to X-ray diffraction, particle size distribution, gravimetric and Fourier transformed infrared spectroscopy analyses. Particle size and crystallite thickness of the formed hBN were seen to increase from about 60 nm and 5 nm at 700 °C to 230 nm and 19 nm at 1200 °C, respectively in NH₃ atmosphere with Na₂CO₃ addition. Highest conversion of boron in the precursor into hBN was achieved as 73.6% when Na₂CO₃ added precursor was reacted at 1200 °C in NH₃. hBN powder with high yield and relatively large particle size was obtained at low temperature such as 1200 °C with Na₂CO₃ addition. Role of Na₂CO₃ addition was suggested to be formation of a sodium borate melt from which hBN crystallized via the reaction of borate and nitrogen ions in the melt. Obtained hBN has the potential for utilization as a clean starting material for synthesis of B or N containing compounds.     

Synthesis and electrical properties of Ce0.8Sm0.2O1.9 ceramics for IT-SOFC electrolytes by urea-combustion technique

Fine Ce_0.8Sm_0.2O_1.9 (SDC) powders with a fluorite cubic phase were prepared using a urea-combustion technique. The sinterability and microstructural evolution of the resulting ceramics were investigated. The results indicate that the ceramics sintered above 1350 °C display relative densities of about 98.5% along with significant grain growth. With respect to their electrical conduction properties, the specimens sintered above 1350 °C exhibit an excellent total ionic conductivity of 0.082 S cm⁻¹ at 800 °C in air. However, when measured in an N₂ + 33.3%H₂ atmosphere, a pronounced Warburg feature appears in the impedance plots of the SDC ceramics, together with a significant increase of the total conductivity at measuring temperatures above 550 °C, due to the introduction of a mixed Ce⁴⁺/Ce³⁺ valence state and the generation of the electronic conduction in the reducing atmosphere.     

Low-temperature synthesis of ZnNb2O6 powders via hydrothermal method

ZnNb₂O₆ powder was successfully synthesized via hydrothermal method with Nb₂O₅ and Zn(NO₃)₂·6H₂O as raw materials and cyclohexane as solvent. Phase composition, morphology, and chemical composition were determined via a combination of XRD, SEM, TEM and EDS techniques. The effects of synthesis temperature and reaction time on phase composition and particle morphology were investigated in this paper. The results showed that fine ZnNb₂O₆ powders could be obtained at a hydrothermal temperature of 190 °C or above under different reaction time.     

Effect of propellant on the combustion synthesized Sr-doped LaMnO3 powders

Lanthanum strontium manganite (LSM) powders of composition La_0.7Sr_0.3MnO₃ are good candidates for cathode application in solid oxide fuel cells. This paper reports the synthesis of LSM powders from nitrate precursors by the combustion method, using two different propellants (urea and glycine) and varying the propellant/nitrate ratio. Thermogravimetric analysis (TGA) revealed two or three decomposition stages of the as-synthesized samples, with complete burn out of organics at about 850–900 °C. X-ray diffraction (XRD) patterns showed formation of only LSM phase for the sample synthesized with excess of urea, whereas SrCO₃ and MnCO₃ phases were also found for the samples prepared from glycine. The powder is better crystallized when a homogeneous gel is formed before burning. The crystallite size calculated using the Scherrer equation is in the range of 15–20 nm. Scanning electron microscopy (SEM) revealed the presence of agglomerates, formed by fine particles of different shapes.     

Synthesis of negative thermal expansion materials ZrW2−xMoxO8 (0 ≤ x ≤ 2) using hydrothermal method

Negative thermal expansion materials ZrW_2−xMo_xO₈ (0 ≤ x ≤ 2) have been successfully synthesized by the reaction of a mixture of ammonium tungstate and ammonium molybdate with zirconium oxynitrate using a hydrothermal method. Effect of substituted ion Mo on the microstructure, α-to-β and cubic to trigonal phase transition in resulting ZrW_2−xMo_xO₈ powders was examined by the XRD experiments. It was found that the structural phase transition temperature decreased slightly with increasing substituted content. The cubic to trigonal phase transition was also influenced by substituted content. The resulting products decomposed to WO₃/MoO₃ and ZrO₂ as temperature increasing when x ≤ 0.5 and while x > 0.5, the cubic phase transited to trigonal phase. The effect of substituted Mo on the morphology of resulting products was also investigated by SEM experiments.     

IBLC effect leading to colossal dielectric constant in layered structured Eu2CuO4 ceramic

Dielectric (ε_r′) studies of phase pure T′-type Eu₂CuO₄ ceramics of two markedly different grain sizes (D), prepared by (i) conventional powder mixing and (ii) citrate complexation-Pechini process, have been carried out in the frequency range 0.1 Hz to 1 MHz, and at temperatures −100 °C to 150 °C. ε_r′ is found to be highly grain size dependent. For the sample with coarse bar-like grains (D²~17×6 μm²) ε_r′ is >10³, and for the finer grain size sample with bimodal distribution (D₁~1 μm, D₂~3 μm) ε_r′ is ~10⁵; for both the samples, high ε_r′ value is nearly frequency independent over 500 Hz≤f<100 kHz and T≥30 °C. The impedance spectroscopy (IS) study has clearly shown that both, the coarse- and the fine-grained samples consist of semiconducting grains and insulating grain boundaries that primarily lead to an internal barrier layer capacitance (IBLC) effect. And thus, manifest colossal dielectric constant (ε_r′>10³) in Eu₂CuO₄ ceramics. The smaller grain size (Pechini) sample, with over an order higher number of grains and grain boundary network, showing over an order higher ε_r′ (~10⁵) compared to the coarse grained one, further endorses the IBLC effect.