Molten Salt Synthesis Technique Research Articles

Molten-Salt Synthesis of O3-Type Layered Oxide Single Crystal Cathodes with Controlled Morphology towards Long-Life Sodium-Ion Batteries.

Sodium layered oxides show great promise as affordable alternatives to lithium layered oxides, but their poor cycle life and air stability limit their practical potential. Micron-scale single crystals with greater packing density and lower surface area can overcome these challenges and improve performance compared to the traditional polycrystalline morphology. Herein, the authors present the synthesis of layered O3-type Na(Ni0.3 Fe0.4 Mn0.3 )O2 single-crystals with greatly improved cycle life and air stability. A molten-salt synthesis technique is adopted with excess sodium hydroxide to obtain platelet-like single crystals. Because the main mechanisms of both capacity fade and air degradation occur as a result of surface reactions at the opening of the sodium diffusion channels, particle morphology is found to be a critical metric for materials performance. More important than particle size or total surface area, the smaller proportion of exposed edge planes in the platelet morphology greatly reduces the amount of harmful surface reactions. Furthermore, the molten-salt method is found to eliminate the need for coprecipitated precursors and even form better morphology, starting from metal oxides instead of coprecipitated hydroxides.

CaSiO3:Eu2+, Er3+, BaSiO3:Eu2+, Er3+ and SrSiO3:Eu2+, Er3+ phosphors: molten salt synthesis, optical and thermal studies

In this study, thermal and optical behaviors of CaSiO3:Eu2+, Er3+, BaSiO3:Eu2+, Er3+ and SrSiO3:Eu2+, Er3+ phosphors synthesized by molten salt synthesis technique are investigated. The doped phosphors activated with Eu2+ and Er3+ ions with varying doping concentrations of 0 mol% to 1 mol% were subjected to structural, optical, and thermal analyses. The powder X-ray diffraction analysis confirms the triclinic structure of CaSiO3: Eu2+, Er3+, orthorhombic structure of BaSiO3: Eu2+, Er3+, and monoclinic structure of SrSiO3: Eu2+, Er3+ phosphors with good homogeneity and phase purity. The morphological and topographical traits are studied via field emission scanning electron microscopy and transmission electron microscopy analyses. These analyses indicate the irregular shape and size of particles along with agglomeration that occurred as a result of prolonged heat treatment processes. The thermal behavior studied by thermogravimetric (TG) analysis confirms the thermal stability of these phosphors with less weight loss occurring while heating the phosphors from room temperature to 700 °C. The study of thermoluminescent spectra of ultraviolet (UV) and gamma-irradiated phosphors confirms deep trapping that occurred in the synthesized phosphors along with second-order kinetics. The trap depth analysis indicates the trapping of electrons in deep trap centers and hence high activation energy (~ 3 eV) is required to vacate the trapped electrons. Doping of conjugate rare earth pair Eu2+ and Er3+ in the host lattice leads to formation of more defects and hence enhances the trapping mechanism. The gamma-irradiated phosphors exhibit increased TL intensity, efficient trapping in much deeper trap centers, and improved fading characteristics in comparison to the UV-irradiated phosphors.

Molten salt synthesis, formation mechanism, and oxidation behavior of nanocrystalline HfB2 powders

Nanocrystalline HfB2 powders were successfully synthesized by molten salt synthesis technique at 1373 K using B and HfO2 as precursors within KCl/NaCl molten salts. The results showed that the as-synthesized powders exhibited an irregular polyhedral morphology with the average particle size of 155 nm and possessed a single-crystalline structure. From a fundamental aspect, we demonstrated the molten-salt assisted formation mechanism that the molten salts could accelerate the diffusion rate of the reactants and improve the chemical reaction rate of the reactants in the system to induce the synthesis of the high-purity nanocrystalline powders. Thermogravimetric analysis showed that the oxidation of the as-synthesized HfB2 powders at 773–1073 K in air was the weight gain process and the corresponding oxidation behavior followed parabolic kinetics governed by the diffusion of oxygen in the oxide layer.

Study of optical behaviour of Eu3+ and Tb3+ doped zirconate perovskite phosphors prepared by molten salt technique

This paper gives a keen insight of the study of spectroscopic and optical behavior of Eu3+ and Tb3+ doped calcium zirconate, barium zirconate and strontium zirconate perovskite phosphors synthesized by molten salt synthesis (MSS) technique. Here, the low temperature MSS technique uses molten chloride salt sodium chloride NaCl that increases solid–solid diffusion rate for creating an active reaction medium. These phosphors were subjected to sintering at a temperature of 950 °C with Eu3+ and Tb3+ ions doping concentration varying from 0 to 2 mol%. The Powder X-ray diffractometry technique was used to study of crystallographic behavior of synthesized phosphors. In order to study the morphological and topographical aspects, field-emission scanning electron microscopy and transmission electron microscopy analyses were done. The Fourier transform infrared analysis generates the fingerprint of the synthesized phosphors confirming the presence of all constituent elements. The photoluminescent (PL) characteristics of synthesized phosphors were studied on exciting CaZrO3:Eu3+, Tb3+ with 225 nm, BaZrO3:Eu3+, Tb3+ with 229 nm and SrZrO3:Eu3+, Tb3+ with 235 nm excitation wavelength. The PL emission spectra show variable emissions over a broad spectral region corresponding to 5D4–7F6, 5D4–7F5, 5D4–7F4 and 5D4–7F3 hypersensitive electronic transitions of Tb3+ ions along with 5D0–7F0, 5D0–7F1 and 5D0–7F2 induced electric dipole and magnetic dipole transitions of Eu3+ ions. These color tunable phosphors with increased PL output confirm their suitability for photonics, optoelectronics and display device applications.

Synthesis and characterization of the ternary metal diboride solid‐solution nanopowders

AbstractThe synthesis of the multi‐component transition‐metal diboride (MeB2) solid‐solution powders has been recently attracting considerable attentions. However, the synthesis of the ternary or more component MeB2 solid‐solution powders has rarely been reported until now. To fabricate the ternary MeB2 solid‐solution powders, herein we utilized two kinds of the ternary MeB2 solid solutions as prototypes, namely (Hf1/3Zr1/3Ti1/3)B2 (HZTB) and (Ta1/3Nb1/3Ti1/3)B2 (TNTB). The formation possibility of HZTB and TNTB was first analyzed by the first‐principles calculations and then we attempted of fabricated them by a simple molten salt synthesis technique. The first‐principles calculations results showed that the mixing Gibbs free energy at room temperature and lattice size difference at 0 K of HZTB and TNTB were (1.666 kJ/mol and 3.146%) and (−3.030 kJ/mol and 1.254%), respectively. This suggested that TNTB solid solution was more prone to being fabricated than HZTB solid solution. The experimental results showed the high purity TNTB solid‐solution nanopowders were successfully synthesized by the molten salt synthesis technique at 1373 K with 30% excessive B as precursors while the HZTB solid solution was not able to be synthesized by the molten salt synthesis technique. The as‐synthesized TNTB solid‐solution nanopowders exhibited the distinguished nanorod morphology with the diameters of 20‐40 nm and lengths of 100‐200 nm. Meanwhile, they possessed the good single‐crystal hexagonal structure and high compositional uniformity from nanoscale to microscale. In addition, their formation mechanism associated to the possible chemical reactions was well interpreted by the thermodynamics analysis.

Molten salt synthesis, characterization, and formation mechanism of superfine (HfxZr1‐x)B2 solid‐solution powders

AbstractHigh‐purity superfine (HfxZr1‐x)B2 solid‐solution powders with various chemical compositions (x = 0.2, 0.5, and 0.8) were successfully synthesized via molten salt synthesis technique at 1423 K using ZrO2, HfO2, and 20% excessive B as precursors within a KCl/NaCl molten salt for the first time. The results showed that the as‐synthesized solid‐solution powders exhibited the irregular polyhedral morphology with the average particle sizes of 170–185 nm and simultaneously possessed a single‐crystalline hexagonal structure and good compositional uniformity from nanoscale to microscale. In addition, their formation mechanisms were well interpreted by analyzing the thermodynamics of the possible chemical reactions in the molten salt.

Facile molten salt synthesis, structural, morphological and optical studies of ASiO3: Eu2+, Er3+ (A = Ca, Ba, Sr) perovskites

This work reports structural, morphological and optical studies of alkaline earth metasilicates ASiO3: Eu2+, Er3+ (A = Ca, Ba, Sr) perovskites synthesized by the facile molten salt synthesis technique. Using the molten chloride salt, NaCl to create an active reaction medium, the phosphors with doping concentration of Eu2+ and Er3+ ions varying from 0 to 2 at.% were synthesized at sintering temperature of 950 °C. The crystallographic analysis done by PXRD confirms the crystalline nature, homogeneity and phase purity of synthesized perovskites. The FESEM and TEM micrographs show the distinct morphology and uniform grain connectivity of the particles. The FTIR spectroscopy set forth the finger print of the molecular activities of the constituent elements of the synthesized phosphors. The optical studies done by PL spectroscopy show distinct emissions of synthesized phosphors with enhanced quantum yields over a wide spectral region resulting from the hypersensitive transitions 2P3/2–4I15/2, 4G7/2–4I15/2, 4G11/2–4I15/2, 4H9/2–4I15/2, 2P3/2–4I11/2 and 4F7/2–4I15/2 of Er3+ ions and crystal field splitting influenced 4f–5d transitions of Eu2+ ions. These emissions confirm the important characteristic of color tunability of the phosphors. The dipole–dipole interaction between rare earth ions crucially amend the optical behavior of synthesized phosphors giving rise to distinct spectroscopic characteristics and enhanced lumen output of the synthesized ASiO3:Eu2+, Er3+(A = Ca, Ba, Sr) perovskites.

Facile in-situ formation of high efficiency nanocarbon supported tungsten carbide nanocatalysts for hydrogen evolution reaction

Electrochemical hydrogen evolution reaction (HER) is one of the key techniques for hydrogen production. Much great effort has been made so far to develop highly efficient HER catalysts to replace expensive precious metals (e.g. Pt). Unfortunately, the synthesis processes were generally not cost-effective and/or scalable. So it is highly desirable to develop a facile technique to enhance HER activity of conventional inexpensive but less active materials. In this work, monodispersed tungsten carbide (WC) nanoparticles (<5 nm) were in-situ formed/anchored on nanosized carbon black (CB) and carbon nanotube (CNT) via a simple low temperature molten salt synthesis technique. Owing to this special hybrid structure, both the exposed surface area of active species and the electrical conductivity of the catalysts were increased effectively, making the catalysts perform considerably better in HER than pure WC and WC based catalysts prepared via other conventional routes. WC nanocrystals in-situ formed/anchored on CNTs showed small onset overpotential (90 mV), low Tafel slope (69 mV dec−1), high current density (93.4 and 28 mA cm−2 at 200 and 300 mV, respectively) and excellent stability (remaining stable even after 3000 cycles). Such a performance is one of the best among those of WC based electrocatalysts developed to date. We demonstrate here significantly improved HER performances of inexpensive tailored WC materials, along with a facile synthesis strategy which could be also readily extended to prepare a range of other types of mono-dispersed nanocatalysts for more potential applications.

Integrated investigation of the Li4Ti5O12 phase stability

The Li4Ti5O12 is applied in lithium ion batteries as anode material, which can be synthesized by various synthesis techniques. In this study, the molten salt synthesis technique at low temperatures, i.e. 350 °C, was applied to synthesize Li4Ti5O12. Surprisingly, the Li4Ti5O12 was not formed according to XRD analysis, which raised question about the stability range of Li4Ti5O12. To investigate the stability of Li4Ti5O12 at low temperatures, the high-temperature calcined Li4Ti5O12 powder was equilibrated in the LiCl-KCl eutectic salt at 350 °C. The result of experiment revealed that the Li4Ti5O12 is not decomposed. Results of ab initio calculations also indicated that the Li4Ti5O12 phase is a stable phase at 0 K. The products of molten salt synthesis technique were then annealed at 900 °C, which resulted in the Li4Ti5O12 formation. It was concluded that the Li4Ti5O12 is a stable phase at low temperatures and the reasons for not forming the Li4Ti5O12 by molten salt technique at low temperature are possibly related to activation energy and kinetic barriers. The Li4Ti5O12 formation energy is also very small, due to the results of ab initio calculations.

Molten salt-directed synthesis method for LiMn2O4 nanorods as a cathode material for a lithium-ion battery with superior cyclability

A molten salt synthesis technique has been used to prepare nanorods of Mn2O3 and single-crystal LiMn2O4 nanorods cathode material with superior capacity retention. The molten salt-directed synthesis involved the use of NaCl as the eutectic melt. The as-synthesized LiMn2O4 nanorods cathode material showed superior electrochemical performance compared to the LiMn2O4 sample obtained via the solid state method. The as-synthesized LiMn2O4 nanorods maintained more than 95% of the initial discharge capacity of 107 mA h g−1 over 100 cycles at a rate of 0.1 C, whereas the LiMn2O4 sample synthesized using the solid state reaction method maintained 88% of the initial discharge capacity of 98 mA h g−1 over 100 cycles at a rate of 0.1 C. Compared to the literature, the molten salt-directed method for the preparation of high-performance LiMn2O4 is simpler and less expensive, with greater potential for industrial scale-up.

Biopolymer-Mediated Inorganic Crystal Growth at Reduced Temperature with Enhanced Kinetics and Morphological Direction

We describe and develop an example of the successful application of a broadly applicable biomimetic growth process to the crystal growth of the complex inorganic quaternary high-temperature superconducting ceramic oxide YBa2Cu3O7−δ. By combining effective biopolymer mediation with established synthetic single crystal growth processes, we leverage a reduction in the required growth temperature from 1045 to 920 °C to enable the successful use of a molten salt synthesis technique in place of the high-temperature solution method commonly employed. Further, the influence of the biopolymer on the seed crystal nucleation provides for morphological control of the synthesis product, offering the potential for designed macromorphologies through biopolymer selection or customization. The method described is directly applicable to other difficult-to-synthesize materials that presently rely on similar conventional techniques, or for which such approaches have been found to fail, thereby providing a novel route to the successful bulk synthesis of complex refractory materials.

Template-free fabrication of pure single-crystalline BaTiO3 nano-wires by molten salt synthesis technique

Abstract Large amounts of BaTiO3 nano-wires have been produced by the molten-salt synthesis method. Powder X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy were used to investigate structure and morphology of the obtained products. Moreover, the current work also proposes synthesis mechanism of BaTiO3 nano-wires when barium nitrate and titanium dioxide were chosen as starting materials. The results revealed that the dissolution rate of the initial precursors was a critical factor refining the shape of the final products. An impedance-type humidity sensor was finally fabricated based on as-prepared BaTiO3 nano-wires and tested with humidity performance providing that BaTiO3 nano-wire is suitable for high-performance humidity sensors.

Molten salt synthesis of YAlO3 powders

In this work, the molten salt synthesis technique was applied to the synthesis of YAlO3 powder using LiCl, NaCl or KCl salt as the flux. YAlO3 powder was synthesized by reacting equimolar amounts of Y2O3 and Al2O3 powders in LiCl salt. The synthesis temperature for YAlO3 using LiCl salt was 1300 °C which is by about 500 °C lower than that in the conventional mixed-oxide method. The synthesized powders have been characterized using powder X-ray diffraction (XRD) analysis and field emission scanning electron microscopy (FE-SEM). The effect of the salt type on the formation of YAlO3 has also been investigated.

Single crystalline nanostructures of giant dielectric calcium copper titanate: a convenient route toward materialization of hard to realize multi-component perovskite nanostructures

Single-crystalline nanostructures of calcium copper titanate, an important giant dielectric quaternary oxide is disclosed for the first time through a convenient molten salt synthesis technique. The technique is not only blessed with high yield but also can readily generate a wide range of morphologies from nanoparticles, nanocubes to nanorods in reproducible way. Most importantly, the as-synthesized nanostructures, irrespective of their shapes, maintain single-crystalline character as it is evident from high-resolution transmission electron microscopic investigation. The present study will pave the way for the development of single-crystalline multi-component perovskite nanostructures which were seem to be extremely hard to realize to date. The possible mechanism on morphological evolution of the as-prepared nanostructures in molten salt is also discussed.

Novel low temperature synthesis and characterisation of hollow silicon carbide spheres

A novel molten salt synthesis (MSS) technique has been used to form in situ silicon carbide (SiC) coatings/shells on spherical carbon black (CB) particles. As-prepared SiC-coated CB particles were oxidised in air for 90 min at 600 °C to remove the residual carbon cores giving the desired hollow SiC spheres (HSS). HSS retained the morphologies and sizes of the CB templates, with an average shell thickness of ∼37 nm. As the MSS of SiC-coated CB was carried out in the molten salt media, the synthesis temperature was reduced to 1150 °C, which is 150 °C lower than required by the conventional vapour–solid reaction technique. Apart from this and the use of cheap and environmentally-friendly raw materials, the MSS technique has several other potential advantages. Firstly, SiC shell thickness could be readily tailored by simply adjusting the Si/CB ratio. Secondly, no special equipment is required and the process should be readily scalable and cost-effective, and finally, the technique could also be used to prepare other types (e.g. other carbides or complex oxides) of hollow spheres. The prepared HSS showed a very high resistance in an acid solution.

Photovoltaic performance of dye sensitized solar cell based on rutile TiO 2 scaffold electrode prepared by a 2 step bi-layer process using molten salt matrices

Dye sensitized solar cells were made on TiO 2 scaffold anodes of rutile particles. These TiO 2 scaffold anodes were grown from rutile seeds by using a molten salt synthesis technique. Different thickness coatings of mixed amorphous titanium hydroxide and NaCl–KCl eutectic salt mixture on the rutile seeds were heat treated at different temperatures. The rutile whiskers of different aspect ratios were grown depending on the growth temperature. The best photovoltaic performances were obtained for the device made from the scaffold of 20–50 nm diameter and 0.5–1 μm length nanowhiskers obtained at 700 °C for 5 h of treatment.

Synthesis of TiO 2 scaffold by a 2 step bi-layer process using a molten salt synthesis technique

TiO 2 scaffolds of anisotropic rutile particles were grown from rutile seeds by using molten salt synthesis techniques. The rutile seeds were either in the form of a separate layer applied on a substrate or a sintered bulk pellet. Mixtures of amorphous titanium hydroxide and salt applied as coatings on the rutile seeds were heat treated. Depending on the morphology of the seed layer, heat treatment temperature, time and salt medium, rutile was grown with different morphologies and microstructures. For NaCl–KCl eutectic salt mixture and heat treatment at 700 °C for 5 h, nano-whiskers of 20–50 nm diameter and 0.5–1 μm length were obtained. For the NaCl salt sample treated at 850 ºC for 20 h, rutile platelets of 2–5 μm thick, 2–10 μm wide and 5–25 μm in length were produced. X-ray diffraction and Raman scattering were used to identify and characterize the rutile phase of the nanowhiskers.

Molten Salt Synthesis and Characterization of Titanium Carbide-Coated Graphite Flakes for Refractory Castable Applications

Titanium carbide (TiC)-coated graphite flakes were prepared using the low-temperature molten salt synthesis technique. Titanium (Ti) particles were mixed with graphite in a mass ratio (Ti/C) between 1/5 and 3/5, and reacted in alkali chloride salts at 650–950°C for 4–8 h. The TiC formation reaction was complete in KCl or the KCl–LiCl eutectic salt after 8 h at 850°C, but not in LiCl or NaCl, indicating that the former were more effective than the latter in accelerating the TiC coating formation. Although various Ti/C ratios were used, the TiC formation reaction was complete in all samples heated for 4 h at 950°C, indicating that the amounts/thickness of TiC coatings could be readily tailored for future castable applications. TiC coatings prepared in KCl or the KCl–LiCl eutectic salt (after 4 h at 950°C or 8 h at 850°C) were of high quality: crack free, homogeneous, and comprising nanosized TiC particles. The coating synthesis process is believed to be dominated by the “template-growth” mechanism.

ME response of cofired trilayer magnetoelectric composites with partial texturing

Magnetoelectric (ME) effect is the combination of two types of materials property such as magnetostriction and piezoelectricity [1–3]. ME effect can be described as the induced electrical polarization under magnetic field or induced magnetization under electric field. The interrelation between ferroelectricity and magnetism allows the magnetic control of ferroelectric properties and vice versa [4–7]. The composite systems are easy to fabricate as compared to the single phase materials, cost effective, and have higher working temperature range [8–10]. Sintered composites fabricated by mixing of piezoelectric and ferromagnetic phases have been widely studied because of its simplicity in synthesis using well-established conventional ceramic processing technique. To enhance the ME coefficient of sintered composite, it is necessary to optimize the composition, microstructural features (grain size, grain orientation), geometry, and sintering parameters. In our previous studies, it have been shown that soft piezoelectric phase (high dielectric and piezoelectric constant), soft magnetic phase (high permeability and low coercivity) [11], large piezoelectric grain size ([1 lm) [12], layered structure (bilayer/trilayer) [13], and post-sintering thermal treatment (annealing and aging) [14] improve the ME property. For synthesizing the cofired trilayer composite, we introduced the noble pressure assisted sintering. We use this process to investigate the effect of grain texturing on ME coefficient of the trilayer composite. Crystallographic texturing of piezoelectric phase can improve the piezoelectric and ferroelectric response by exploiting the anisotropy of electrical properties. Results have been reported in literature on the effect of texturing in both lead-based and lead-free materials including BTO, PMN-PT, NBT-BT, and KNN [15–18]. The goal of this study was to implement the known texturing process in heterogeneous system consisting of varying crystal structure. This will allow developing novel hybrid materials that exhibit maximum response. As a simple rule, the rhombohedral phase oriented along the h100i direction provides higher piezoelectric response. Texturing can be accomplished through various processes including, hot pressing (HP), templated grain growth (TGG) [19], and reactive template grain growth (RTGG) [15]. The advantage of TGG over HP is that the process is conducted using steps similar to that of conventional ceramic processing except with added seed material. In HP there is a chance of abnormal grain growth. Hence in this study, the TGG technique was adopted to synthesize the textured ME coefficient. The texturing process consists of two important steps (i) seed fabrication and (ii) seed alignment in the matrix [20]. To fabricate the seed (template), molten salt synthesis technique was utilized. In this respect, BaTiO3 was chosen as the seed material since it can be grown in large size using Remeika process [21]. This synthesis process consists of heating the raw materials (BTO covered with KF salt in Pt crucible) in the temperature range of 1150– 1225 C (±5 C) for 8–12 h and then slowly cooling down to the room temperature. The crystals were separated from R. A. Islam (&) Materials Science and Engineering, University of Texas at Arlington, Arlington, TX 76019, USA e-mail: rashed_ms@yahoo.com

Processing–Property Relations in Grain‐Oriented Lead Metaniobate Ceramics Fabricated by Layered Manufacturing

Highly grain‐oriented lead metaniobate (PbNb2O6; PN) ceramics were prepared by a layered manufacturing (LM) process. This process has enabled us to obtain a net‐shaped piezoelectric ceramic component with grain orientation of f∼89%, and improvement in electromechanical properties. The LM feedstock (filament) includes equiaxed (matrix; 90 vol%) and anisometric PN templates (10 vol%), both dispersed uniformly in a thermoplastic binder. The needle‐like PN templates were synthesized by molten salt synthesis technique, while the equiaxed PN powder was prepared by conventional ceramic processing methods. The processing conditions were studied and optimized to obtain orthorhombic phase fine powder and anisometric templates. Samples were obtained through layer‐by‐layer deposition of the filament through a small diameter (500 μm) nozzle. After binder removal, the PN samples were sintered in a temperature range of 1150°–1300°C for 1 h. SEM observation revealed strong grain orientation perpendicular to the deposition direction. Relative permittivity at the Curie point (Tc: 560°C) was 18 100 and 14 600 for the LM and random polycrystalline samples, respectively. Improved properties in piezoelectric figure of merit by 71%, d33 by 23%, and g33 by 31% were observed in the grain‐oriented samples. Remnant polarization also showed about 80% improvement, increasing from 4.5 to 8.1 μC/cm2 for the grain‐oriented LM samples.