Molten salt synthesis of La2Zr2O7 ultrafine powders

Ultrafine powders of pyrochlore-type La₂Zr₂O₇ were synthesized via a simple molten salt mediated process using zirconium oxide and lanthanum oxide as raw materials, and sodium chloride, potassium chloride and sodium fluoride to form a reaction medium. The effects of reaction temperature, salt/reactant ratio and salt type on the La₂Zr₂O₇ formation were investigated. Among the three attempted salt assemblies (KCl–LiCl, Na₂CO₃–K₂CO₃, and NaCl–KCl–NaF), NaCl–KCl–NaF showed the best accelerating effect on the La₂Zr₂O₇ formation. At a given temperature, the La₂Zr₂O₇ content in the final products increased with the increase in the salt amount. Phase pure submicron sized La₂Zr₂O₇ ultrafine powders were obtained after 3 h firing at 1100 °C with the salt/reactant weight ratio of 5:1 or at 1200 °C with salt/reactant weight ratio of 3:1. The synthesis temperature (1100 °C) was much lower than that required by the conventional solid-state mixing method or a wet chemical method. The “dissolution–precipitation” mechanism had dominated the synthesis process.     

Low temperature synthesis of TaB2 nanorods by molten‐salt assisted borothermal reduction

TaB₂ powders were synthesized by a molten‐salt assisted borothermal reduction method at 900°C‐1000°C in flowing argon using Ta₂O₅ and amorphous B as starting materials. The results indicated that the presence of liquid phase, such as B₂O₃ and NaCl/KCl, accelerated the mass transfer of reactant species and resulted in the complete finish of the reaction at low temperatures. The obtained TaB₂ powders exhibited a flow‐like shape assembled from nanorods grow along [001] direction or c‐axis. The morphology of the synthesized TaB₂ powders could be tailored by the amount of B₂O₃ or NaCl/KCl.     

Synthesis and characterization of polyrhodanine/nickel ferrite nanocomposite with an effective and broad spectrum antibacterial activity

Nickel ferrite nanoparticles were successfully synthesized via a co-precipitation approach, and then polyrhodanine/nickel ferrite nanocomposite (PRh/NiFe₂O₄) as an antimicrobial agent was fabricated by a chemical polymerization method. The synthesized NiFe₂O₄ nanoparticles and PRh/NiFe₂O₄ nanocomposite were chemically, magnetically and morphologically characterized using FTIR, FESEM, DLS, VSM and XRD techniques. The FESEM analysis showed that the NiFe₂O₄ nanoparticles had a polygon structure with an average diameter of 50 nm. According to disc diffusion as well as MIC and MBC tests, the PRh/NiFe₂O₄ nanocomposite had better antibacterial effects on killing Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli in comparison with the NiFe₂O₄ nanoparticles.     

Molten Salt Synthesis of Bismuth Ferrite Nano‐ and Microcrystals and their Structural Characterization

Bismuth ferrite nano‐ and microcrystals were prepared by a facile molten salt technique in two kinds of molten‐salt‐based systems (NaCl–KCl and NaCl–Na₂SO₄). In the NaCl–KCl salt system, a systematic study indicating the effects of process parameters (e.g., calcination temperature, holding time as well as the molten salt ratios) on the bismuth ferrite formation mechanism and structural characteristics is reported. The results show that almost pure phase BiFeO₃ powders with minimum impurity phase of Bi₂Fe₄O₉ were synthesized at temperatures of 700°C–800°C, whereas high calcination temperature (e.g., 900°C) resulted in the formation of almost pure phase Bi₂Fe₄O₉ powders. The prolonged holding time increased the particle size via the Ostwald ripening mechanism; however, there was little effect on the particle morphology. Similar phenomenon occurred as increasing the molten salt ratios. In the NaCl–Na₂SO₄ salt systems, it is found that low NP‐9 (nonylphenyl ether, NP‐9) surfactant content (0–5 mL) led to the formation of almost pure phase BiFeO₃ powders, whereas high NP‐9 surfactant content (e.g., 20 mL) resulted in pure phase Bi₂Fe₄O₉ powders. The average particle size of the BiFeO₃ powders was decreased as increasing the NP‐9 surfactant content, whereas their morphologies did not change significantly. Because of the simplicity and versatility of the approach used, it is expected that this methodology can be generalized to the large‐scale preparation of other important transitional metal oxides with controllable sizes and shapes.     

In situ synthesis of Al2O3–SiC powders via molten-salt-assisted aluminum/carbothermal reduction method

Al₂O₃–SiC composite powder (ASCP) was successfully synthesized using a novel molten-salt-assisted aluminum/carbothermal reduction (MS-ACTR) method with silica fume, aluminum powder, and carbon black as raw materials; NaCl–KCl was used as the molten salt medium. The effects of the synthesis temperature and salt-reactant ratio on the phase composition and microstructure were investigated. The results showed that the Al₂O₃–SiC content increased with an increase in molten salt temperature, and the salt–reactant ratio in the range of 1.5:1–2.5:1 had an impact on the fabrication of ASCP. The optimum condition for synthesizing ASCP from NaCl–KCl molten salt consisted of maintaining the temperature at 1573 K for 4 h. The chemical reaction thermodynamics and growth mechanism indicate that the molten salt plays an important role in the formation of SiC whiskers by following the vapor-solid growth mode in the MS-ACTR treatment. This study demonstrates that the addition of molten salt as a reaction medium is a promising approach for synthesizing high-melting-point composite powders at low temperatures.     

Low‐Temperature Synthesis of Nanocrystalline NbB2 Powders by Borothermal Reduction in Molten Salt

Nanocrystalline NbB₂ powders were successfully prepared by borothermal reduction in molten salt at 800°C–1000°C. Due to the more homogeneous mixing and more rapid diffusion of species in the liquid state than in the solid state, the synthesis temperature of pure NbB₂ phase was greatly decreased by the presence of molten NaCl/KCl salt. The NbB₂ powders synthesized at 1000°C had the largest specific surface area of 27.09 m²/g and the lowest equivalent average particle size of 32 nm, respectively.     

Low‐temperature molten salt synthesis of forsterite powders with controllable morphology

Forsterite powders with controllable morphology were synthesized using oxides as raw materials in NaCl–KCl molten salt media. The effects of MgO/SiO₂ ratio, calcining temperature, and salt/oxide ratio on the phase composition and morphology of the powders are investigated. The results indicate that single‐phase forsterite powders can be synthesized from a mixture of MgO and SiO₂ with a MgO/SiO₂ molar ratio of 2:1.3 at 700°C. With the increase in calcining temperature, the powders obtained changes from an irregular to a columnar morphology. In addition, the morphology of the forsterite powders produced can also be controlled by altering the salt/oxide ratio.     

Synthesis and Characterization of NiFe2O4 Nanoparticles via Solid‐State Reaction

NiFe₂O₄ nanoparticles were synthesized via solid‐state reaction. The effects of the reaction parameters, namely dispersant content, calcination temperature and annealing time on particle size and morphology were investigated. Magnetic properties of NiFe₂O₄ nanoparticles were tested. The formation mechanism of nanoparticles by solid‐state reaction was also discussed. The results indicated that the synthesized nickel ferrite particles were of nanometer size with different morphologies by slight variation of the reaction conditions. The solid‐state reaction technique is a simple, convenient, inexpensive and effective preparation method of NiFe₂O₄ in high yield.     

Impedance spectroscopic studies on NiFe2O4 with different morphologies: Microstructure vs. dielectric properties

NiFe₂O₄ powders were prepared by combustion synthesis, polyol-mediated and sol–gel methods. Morphological characterization of sintered disks was carried out using scanning electron microscopy (SEM). Dielectric properties of NiFe₂O₄ synthesized by different routes were investigated over the frequency range of 100 Hz–5 MHz at room temperature. A difference in dielectric constant (ε_r) and dissipation factor (tan δ) of NiFe₂O₄ samples obtained by different synthesis methods has been observed. The observed dissimilarity in the behavior originates from the variation in the microstructure of the samples that is evident in the Cole–Cole plot results.     

Molten salt synthesis of zinc aluminate powder

ZnAl₂O₄ powder was synthesised by reacting equimolar ZnO and Al₂O₃ powders in alkaline chlorides (LiCl, NaCl or KCl). Formation of ZnAl₂O₄ started at about 700 °C in LiCl and 800 °C in NaCl and KCl. With increasing temperature, the amounts of ZnAl₂O₄ in the resultant powders increased with a concomitant decrease of ZnO and Al₂O₃. ZnAl₂O₄ powder was obtained by water-washing the samples heated for 3 h at 1000 °C (LiCl) or 1050 °C (NaCl and KCl). ZnAl₂O₄ formed in situ on Al₂O₃ grains from the surface inwards. The synthesised ZnAl₂O₄ grains retained the size and morphology of the original Al₂O₃ powders, indicating that a template formation mechanism dominated formation of ZnAl₂O₄ by molten salt synthesis.     

Low‐temperature synthesis of ultrafine TiB2 nanopowders by molten‐salt assisted borothermal reduction

The synthesis of TiB₂ nanopowders arouses considerable interests due to its importance for implementing the extensive applications of TiB₂ ceramic. Herein, the high‐purity ultrafine TiB₂ nanopowders were successfully synthesized via a molten salt assisted borothermal reduction technique at a relatively low temperature of 1173 K using TiO₂ and B powders as precursors within a KCl/NaCl salt. The results showed that the as‐obtained TiB₂ nanopowders possessed a polycrystallinity structure, and their specific surface area and equivalent average particle size were 33.18 m²/g and 40 nm, respectively. This study provides a new low temperature synthesis technique of TiB₂ nanopowders.     

Synthesis of CoFe2O4 powders with high surface area by solution combustion method: Effect of fuel content and cobalt precursor

High surface area cobalt ferrite (CoFe₂O₄) powders were synthesized by solution combustion method. The dependence of the adiabatic temperature and the released gases during combustion reaction on the fuel content and cobalt precursor type, cobalt nitrate and cobalt acetate, was thermodynamically calculated. Thermal analysis, infrared spectroscopy, X-ray diffractometry, nitrogen adsorption–desorption, electron microscopy and vibrating sample magnetometer were used for investigation of the phase evolution, surface areas, morphology and magnetic properties of the synthesized CoFe₂O₄ powders. The specific surface area decreased from 285.4 to 35.7 m²/g with increasing of fuel to oxidant molar ratio, ϕ, from 0.5 to 1.25 for the cobalt nitrate precursor, while the maximum surface area of 182.1 m²/g was attained at ϕ=1 for the cobalt acetate precursor. The synthesized CoFe₂O₄ powders from the cobalt nitrate precursor exhibited the higher saturation magnetization and coercivity on account of the higher purity and crystallinity.     

Molten-salt-assisted combustion synthesis of B4C powders: Synthesis mechanism and dielectric and electromagnetic wave absorbing properties

A novel electromagnetic wave (EMW) absorber was prepared by combustion synthesis. Boron carbide (B₄C) powders with different grain sizes using a molten-salt-assisted combustion technique with B₂O₃, CB (carbon black), and Mg powders as starting materials, and NaCl as an additives. The effects of the NaCl content on the phase compositions and the microstructure of the products were characterized. A combustion front quenching method was used to elucidate the mechanism for the B₄C powders synthesis. The dielectric, and EMW absorbing properties in the X-band were also investigated. The results showed that the addition of NaCl significantly reduced the grain size of B₄C powders. Nanoscale B₄C powders with cubic polyhedral structures were synthesized using 6 wt% NaCl (labeled as N-6). According to the quenching test results can be obtained that the first step in the combustion synthesis was melting B₂O₃ into a glassy substance. At the same time, Mg melted and formed a liquid pool into which the NaCl dissolved, followed reduction of the B₂O₃ to B. The formed B eventually reacted with CB to form B₄C, and the B₄C particles precipitated from the matrices. The N-6 sample exhibits optimal dielectric and EMW absorbing properties, because of a high specific surface area that enhances interfacial and space charge polarization.     

Preparation of Giant Dielectric CaCu3Ti4O12 Ceramics via the Molten Salt Method from NaCl Flux

CaCu₃Ti₄O₁₂ (CCTO) powders have been synthesized by the molten salt method using NaCl as the flux. The effects of the synthesis temperature on their crystal structures and micromorphologies were investigated in this study. A new secondary phase, Na₂Ti₆O₁₃, was found in the as‐synthesized powders when the temperature exceeds 800°C. Na₂Ti₆O₁₃ and CuO secondary phases result in a significant increase in the dielectric constant and a certain increase in the dielectric loss. 800°C is a suitable synthesis temperature to prepare CCTO ceramics with relatively good dielectric performance using NaCl as the molten salt.     

Effect of TiN content on properties of NiFe2O4-based ceramic inert anode for aluminum electrolysis

NiFe₂O₄-based ceramic inert anodes for aluminum electrolysis doped with various TiN nanoparticles were prepared by a two-step cold-pressing sintering process to investigate how TiN affected the sintering behavior and properties of the composites. The differential scanning calorimetry-thermogravimetry (DSC-TG), X-ray diffraction (XRD), and microstructure analysis results indicated that the Ti and N were evenly distributed in the NiFe₂O₄ matrix to form a solid solution. The maximum linear shrinkage and linear shrinkage rate were enhanced with the increase of TiN nanoparticles contents, and the sintering activation energy of initial stage was lowered from 382.63 to 279.58 kJ mol⁻¹ with the TiN nanoparticles additive range from 0 to 9 wt%. When the content of TiN nanoparticles was 7 wt%, the relative density, bending strength, and elastic modulus reached their maximum values of 97.24%, 73.88 MPa, and 3.77 GPa, respectively, whereas the minimum static corrosion rate of NiFe₂O₄-based ceramic of 0.00114 g cm⁻² h⁻¹ was obtained, mainly attributed to the relatively dense and stable microstructure. The electrical conductivity of NiFe₂O₄-based ceramics presented a clear ascending trend with increasing TiN nanoparticles content and elevated temperature, attributed to the increased concentration and migration rate of carrier.     

Effect of fuel type on the microstructure and magnetic properties of solution combusted Fe3O4 powders

Porous magnetite (Fe₃O₄) powders were synthesized by solution combustion method using the glycine and urea at different fuel to oxidant ratios (ϕ). The combustion behavior depended on the fuel type as characterized by thermal analysis. The structure and phase evolution investigated by X-ray diffraction method showed nearly single phase Fe₃O₄ powders which were achieved only by using the glycine fuel at ϕ=1. The specific surface area and porous structures of the as-combusted Fe₃O₄ powders were characterized by N₂ adsorption-desorption isotherms and scanning electron microscopy, respectively. The surface area using the glycine fuel (62.6 m²/g) was higher than that of urea fuel (42.5 m²/g), due to different combustion reactions. Magnetic properties of the as-combusted powders were studied by vibration sample magnetometry which exhibited the highest saturation magnetization of 74 emu/g using the glycine fuel at ϕ=1 on account of its high purity and large crystallite size.     

Synthesis of photoluminescent polycrystalline SiC nanostructures via a modified molten salt shielded method

Polycrystalline SiC (3C-, 6H-, 24R- and 27H-) nanostructures are synthesized via a modified molten salt shielded synthesis method (m-MS³) in open air using Si and carbon black as the starting materials. The influences of salt species and their amount, and the sintering temperature, are discussed and optimized. Well crystalline SiC nanopowders composed of bountiful microstructures (nanoparticles, nanowires, nanosheets and nanoblocks) are successfully synthesized by m-MS³ at 1250 °C, with KCl and NaCl as the shielding salt. The polycrystalline SiC powders showed excellent photoluminescence property at an excitation wavelength of 330 nm and relatively small band gaps of 2.57–2.74 eV, which are quite attractive among reports for SiC-based materials. The investigation in this paper may provide a prototype strategy for protection-free synthesis of nanostructured SiC powders applicable for ultraviolet luminescence devices.     

Preparation of in situ grown silicon carbide whiskers onto graphite for application in Al2O3-C refractories

C-SiC composite powders were prepared by salt-assisted synthesis from Si powders, graphite, and a molten salt medium (NaCl and NaF) with the molar ratio of Si/C =?1/2 at 1300?°C for 3?h. After the C-SiC composite powders part and complete replacement of the graphite, the mechanical properties, oxidation resistance and slag-corrosion resistance of the Al₂O₃-C materials were studied by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), as well as with dedicated equipment. The results indicated that SiC whiskers, with lengths of 10–50?nm, formed on the surface of the flake graphite, and the activation energy of oxidation of the C-SiC composite powder increased by 45.72?kJ?mol^?1 as compared to that of flake graphite. Furthermore, the decarburization area and slag erosion area of the Al₂O₃-C material decreased after 3?wt% of C-SiC composite powder was substituted for the flake graphite. Meanwhile, the cold modulus of rupture was maintained when 3?wt% of C-SiC composite powder was added. This improved both the oxidation and slag resistance of the Al₂O₃-C materials.     

Properties of NiFe2O4 ceramics from powders obtained by auto-combustion synthesis with different fuels

Nickel ferrite (NiFe₂O₄) powders derived by auto-combustion synthesis using three different fuels (citric acid, glycine and dl-alanine) have been characterized. The sintering behavior of ceramics using these powders has been compared. Oxygen balance (OB) setting for the chemical reaction is found to regulate the combustion reaction rate. A rapid reaction rate and a high flame temperature are achieved with dl alanine fuel yielding single phase NiFe₂O₄ powder in the as-burnt stage, whereas powders derived with citric acid and glycine fuels show poor crystallinity and necessitate post-annealing. The powder particles are largely agglomerated with a non-uniform distribution in shape and size, and the average particle size is estimated in the range ~ 54–71 nm. Powders derived from dl-alanine fuel show better phase purity, smaller crystallite size, larger surface area and superior sintering behavior. Additional Raman modes discerned for dl-alanine derived powder support a 1:1 ordering of Ni²⁺ and Fe³⁺ at the octahedral sites relating to microscopic tetragonal P4₁22 symmetry expected theoretically for the formation of NiFe₂O₄ with inverse spinel structure. Microstructure of sintered ceramics depends on the precursor powders that are used and sintering at 1200 °C is found to be optimum. Citric acid and glycine derived powders yield high saturation magnetization (M_s~47–49 emu/g), but poor dielectric properties, whereas dl-alanine derived powders yield ceramics with high resistivity (~3.4×10⁸ Ω cm), low dielectric loss (tan δ~0.003 at 1 MHz) and high magnetization (46 emu/g). Dielectric dispersion and impedance analysis show good correlation with the changes in the ceramic microstructure.