NaCl KCl Flux Research Articles

Hexagonal Sodium Lutetium Fluoride Microstructures: Facile and Large-Scale Synthesis, Growth Mechanism and Multicolour Emissions

Highly crystalline β-NaLuF4:Ln3+ microstructures were successfully prepared via a mixed NaCl-KCl flux cooling approach. The chemical reaction process and the formation mechanism have been carefully investigated by XRD, SEM, TEM and PL characterizations. Interestingly, as the reaction time is prolonged, the crystalline structure of NaLuF4 changes from α-phase to β-phase, while the granule shape transfers from nanoparticles to solid microsheets then to porous microsheets. By altering the kinds of doped lanthanide ions, upconverting and downconverting luminescence with multi-colour outputs (red, green and blue) can be realized in β-NaLuF4:Ln3+ (Ln = Yb, Er, Tm, Ce, Tb and Eu). This strategy is not only expected to meet the ever-increasing commercial demand, but also offers an alternative in synthesizing rare earth fluorides.

Chloride flux growth of crystalline strontium niobates and nitridation to perovskite SrNbO2N

Highly crystalline (110) layered perovskite Sr2Nb2O7, (111) layered perovskite Sr5Nb4O15 and complex perovskite Sr4Nb2O9 were prepared by NaCl-KCl flux growth method from SrCO3 and Nb2O5. This flux synthesis achieves single strontium niobate phase in contrast to mixed niobates from the solid state reaction with the same heating parameters. A little excess of Sr source was found to be required for the synthesis of Sr5Nb4O15 and Sr4Nb2O9 at elevated temperature due to slight evaporation. The three strontium niobates were converted to perovskite SrNbO2N via thermal ammonolysis under NH3 flow at 900°C. Post-wash treatment was performed to remove the byproduct SrO. This makes additional nanopores in SrNbO2N in the cases of Sr5Nb4O15 and Sr4Nb2O9, and results in increasing surface areas of SrNbO2N with Sr:Nb ratios in the precursors (from 9.9 to 19.8 and 35.5m2/g). On the other hand, the UV–Vis diffusion reflectance spectra reveal decreasing light absorption by defects in SrNbO2N in this order. This suggests fewer low-valent Nb defects in SrNbO2N prepared from precursor with higher Sr:Nb ratio. SrNbO2N prepared from Sr4Nb2O9 would be advantageous for applications that require high surface area and low defect density of the material.

Synthesis and dielectric properties of Na0.5Bi0.5Cu3Ti4O12 ceramic by molten salt method

Na0.5Bi0.5Cu3Ti4O12 (NBCTO) powder was prepared by molten salt method at 700, 750, 800, and 850 °C in NaCl–KCl flux salts, respectively. X-ray diffraction data revealed that the main NBCTO phase of powder was synthesized at a low temperature of 700 °C for 2 h in NaCl–KCl flux, which was reduced by about 250 °C compared with the conventional solid-state reaction method. The evolution of the microstructure was observed by scanning electron microscopy, and the dielectric properties of NBCTO ceramics affected by sintering temperature and sintering time were studied in detail in this paper. The complex impedance plots were also employed to analyze the dielectric properties of NBCTO ceramics. The average grain size of the sintered ceramic increased with the increase in sintering temperature, which lead to the increased dielectric constant of the NBCTO ceramic, whereas the sintering time has affected the dielectric constant slightly. A high dielectric constant of more than 104 and a low loss tangent (tanδ) of 0.06 (at 10 kHz) were obtained for the NBCTO ceramic sintered at 1040 °C for 12 h.

Growth of Individual, Highly Oriented LiFePO4 Crystals on a SUS Substrate Using NaCl–KCl Flux Coating

Olivine-type LiFePO4 has been an interesting material in the development of catalysts and lithium-ion rechargeable batteries. In this study, we propose a new method for growing thin films of LiFePO4 crystals directly on SUS substrates using flux coating. Molten NaCl–KCl flux promoted the formation of individual LiFePO4 crystals with well-defined facets surrounded by {100}, {110}, and {201} faces and highly oriented along the [100] direction. The LiFePO4 crystals attached tightly to the SUS substrate, implying that crystal growth began directly at the SUS surface via heterogeneous nucleation, driven by supersaturation in the NaCl–KCl flux. We further demonstrated the formation of LiFePO4 crystal films using an infrared light–heating furnace system, which reduced the fabrication time from several hours to less than 30 min. Rapid heating and cooling effectively suppressed the corrosion of the SUS substrate caused by the NaCl–KCl flux.

Spectroscopic Investigation on BaSO4:(Mn6+, Mn5+) Crystal

BaSO4:(Mn6+, Mn5+) crystal was grown using MnCO3 as manganese source at high temperature from 650 to 750°C by the NaCl-KCl flux method in air and its spectroscopic properties were investigated. The experimental results indicate that Mn6+ and Mn5+ ions coexisted in the crystal. A broadband emission band peaking at ∼1060 nm due to the 2T2 → 2E transition of Mn6+ ion and a sharp emission band peaking at ∼1183 nm due to the 1E → 3A2 transition of Mn5+ ion were observed. Mn-doped BaSO4 single crystal may be a promising candidate as tunable near-infrared solid-state laser material.

Flux growth of honeycomb-like β-NaYF4:Yb3+, Er3+/Tm3+ crystals with multicolor upconversion luminescence

Highly crystalline NaYF4:Ln3+ (Ln=Yb, Er, Tm) crystals with honeycomb structure were successfully synthesized for the first time by using a mixed NaCl–KCl flux cooling method at a holding temperature of 800°C for 2h in air. The results indicated that the crystals of NaYF4:Ln3+ belonged to hexagonal system and three-dimensional open pores had an average diameter of 1μm interconnected with each other. Upon 980nm laser diode excitation, yellow green and sky blue upconversion fluorescences of β-NaYF4:20%Yb3+, 2%Er3+ crystals and β-NaYF4:20%Yb3+, 0.5%Tm3+ crystals, which was caused by two- or three-photon upconversion processes, was observed clearly.

Influence of calcium oxide on alumina phase transition and morphology development in NaCl–KCl flux

Plate-like α-Al2O3 particles are used as reinforcements of metals, resins or ceramics to improve their mechanical properties, such as elastic modulus, toughness and strength. Ceramic platelets are easier to disperse into matrix phase compared with whiskers, short fibres or nanoparticles. The well developed plate-like α-Al2O3 particles were obtained in NaCl–KCl flux by sintering at 1000°C for 4 h using Al(OH)3 powders as starting materials. The influences of calcium oxide on alumina phase transition and morphology development were focused. When calcium oxide was added, the phase of the sample was κ-Al2O3, and plate-like particles were not observed by firing at 1000°C for 4 h. The α-Al2O3 and H–Al2O3 phases were obtained, but plate-like particles were hardly observed, when the sample was sintered at 1200°C for 4 h.

Molten salt synthesis of α-Al 2O 3 platelets using NaAlO 2 as raw material

α-Al 2O 3 platelets were prepared by a molten salt synthesis method when NaAlO 2 was used as raw material. The effects of the stirring rate during the gel preparation, heating temperature, type and addition amount of molten salts, addition of plate-like α-Al 2O 3 seeds, additives such as TiOSO 4 and Na 3PO 4·12H 2O on the morphology of α-Al 2O 3 were studied. High stirring rate during the gel preparation and high heating temperature not only help to restrain the overlapping of α-Al 2O 3 platelets, but also improve the size distribution. When the heating temperature increases to 1200 °C, most of α-Al 2O 3 platelets are hexagonal in its morphology, and the size of platelets becomes relatively uniform. When Na 2SO 4–K 2SO 4 flux is used instead of NaCl–KCl flux, it is easy to obtain α-Al 2O 3 platelets with a big size. When the molar ratio of salt to final Al 2O 3 powders increases to 4:1, most of α-Al 2O 3 platelets are hexagonal, and the overlapping of powders is inhibited. The addition of a small amount of plate-like seeds has a significant effect on the size of α-Al 2O 3 platelets. With the increase of seed amount, the diameter of α-Al 2O 3 platelets tends to decrease. The addition of 5.45 wt.% TiOSO 4 results in the formation of hexagonal α-Al 2O 3 platelets with an average diameter of 5.1 μm and an average thickness of 1.4 μm. Thin α-Al 2O 3 platelets with a discal shape are obtained owing to the co-addition of 0.51 wt.% Na 3PO 4·12H 2O and 3 wt.% TiOSO 4.

Ca(Zn1/3Nb2/3)O3 Ceramic Prepared by Molten Salt Synthesis Method

Ca(Zn1/3Nb2/3)O3 powders were synthesized by molten salt synthesis method in NaCl–KCl flux at a low temperature of 700°C for 2 h, which is lower than that of the conventional solid-state reaction. This simple process involved mixing of the raw materials and salts in a certain proportion. Subsequent calcining of the mixtures led to Ca(Zn1/3Nb2/3)O3 powders at 700°C. X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques were used to characterize the phase and morphology of the as-prepared Ca(Zn1/3Nb2/3)O3 powders, respectively. After being sintered at 1200°C for 2 h, the densified Ca(Zn1/3Nb2/3)O3 ceramics with good microwave dielectric properties of ϵ r = 33.9, Q × f = 12790, and τ f = −45 ppm/°C have been obtained.

Microwave dielectric properties of Ba5Nb4O15 ceramic by molten salt method

Ba5Nb4O15 powders were synthesized by molten-salt method in NaCl–KCl flux at a low temperature of 650–900 °C for 2 h, which is lower than that of the conventional solid-state reaction. This simple process involved mixing of the raw materials and salts in a certain proportion. Subsequent calcination of the mixtures led to Ba5Nb4O15 powders at 650–900 °C. XRD and SEM techniques were used to characterize the phase and morphology of the fabricated Ba5Nb4O15 powders, respectively. After sintering at 1,300 °C for 2 h, the densified Ba5Nb4O15 ceramics with good microwave dielectric properties of er = 39.2, Q × f approximated as 27,200 GHz and τf = 72 ppm/°C have been obtained.

Morphology-Controlled Synthesis of Barium Titanate Nanostructures

This study reports the synthesis of morphology-controlled BaTiO(3) nanostructures such as spherical, cube-shaped and rod-shaped BaTiO(3) using a molten-salt synthesis method. This method synthesized products from a reaction of BaO/BaCO(3) and TiO(2) with a eutectic mixture of NaCl-KCl flux at 700 degrees C for 1 h. The experiment used powder X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy to investigate the structure and morphology of the products. Moreover, the current work also provides a proposed synthetic mechanism of BaTiO(3) in the molten salt to illustrate the in situ transformation mechanism of BaTiO(3) nanostructures in the reaction. The results of the study revealed that the initial shape of the titania and the dissolution rate of the initial precursors critically determine the shapes of the final products.

Molten-salt synthesis and characterization of Bi-substituted yttrium garnet nanoparticles

Bi-substituted yttrium iron garnet (Bi-YIG, Bi 1.8Y 1.2Fe 5O 12) nanoparticles were successfully synthesized by molten-salt method in NaCl–KCl flux at 650 °C. Subsequently, X-ray powder diffraction, scanning electron microscopy, dynamic light scattering and vibrating sample magnetometer tests were used to characterize the phase, morphology, size distribution and magnetic properties of the as-prepared Bi-YIG nanoparticles, respectively. Moreover, the transparence and Faraday rotation of the PMMA slices filled with Bi-YIG nanoparticles were investigated by ultraviolet–visible spectrophotometer and Faraday rotation meter, respectively.

Low temperature synthesis of Ba 0.70Sr 0.30TiO 3 powders by the molten-salt method

Pure perovskite phase Ba 0.70Sr 0.30TiO 3 (BST) powders were successfully synthesized by molten-salt method in NaCl–KCl flux at a low temperature of 850 °C for 2 h, which is 300 °C lower than that of the conventional solid-state reaction. This simple process involved mixing of the raw materials and salts in a certain proportion. Subsequent calcining of the mixtures led to BST powders at 800–900 °C. XRD and SEM techniques are used to characterize the phase and morphology of the fabricated BST powders, respectively.

Synthesis of plate-like α-Al 2O 3 single-crystal particles in NaCl–KCl flux using Al(OH) 3 powders as starting materials

Plate-like α-Al 2O 3 single-crystal particles were successfully synthesized in NaCl–KCl flux using Al(OH) 3 powders as starting materials, and the influence of pre-calcining of Al(OH) 3 powders on the phase formation and morphology of α-Al 2O 3 powders was focused. When Al(OH) 3 powders are used as starting materials, the synthesized product at 900 °C is mainly composed of α-Al 2O 3 and κ-Al 2O 3, and most synthesized particles show alveolate morphology. At 1100 °C, single-phase α-Al 2O 3 powders are developed, in which there are many aggregations of intensively bound plate-like particles. In contrast, using porous amorphous Al 2O 3 powders obtained by pre-calcining Al(OH) 3 powders at 550 °C for 3 h as the starting material, plate-like α-Al 2O 3 single-crystal particles can be well developed above 900 °C. The reason of the influence of pre-calcining of Al(OH) 3 powders on the phase formation and morphology of α-Al 2O 3 powders is also discussed in the paper.

Effect of neodymium substitution on the growth of single crystalline Bi 4Ti 3O 12 particles

Neodymium (Nd)-substituted Bi 4Ti 3O 12 single crystalline particles are prepared by molten-salt synthesis in NaCl–KCl flux using amorphous Bi 4− x Nd x Ti 3O 12 precursors. X-ray diffraction results indicate that pure bismuth titanate phase is obtained in molten-salt flux and the unit-cell of bismuth titanate shows a slight shrinkage with Nd substitution. Well-dispersed single crystalline particles with a plate-like morphology are revealed by SEM images. The average particle sizes decrease with increasing Nd content, which may be associated with the alteration of surface energy of single crystalline platelets by the substitution of Nd 3+ ions into Bi 4Ti 3O 12 lattice.

Single-crystal growth of Tl 2Ru 2O 7 pyrochlore using high-pressure and flux method

Single crystals of the thallium ruthenium pyrochlore have been grown by flux method under high oxygen pressure. The growth conditions were determined by direct observations using in situ powder X-ray diffraction (XRD) method under high pressure and high temperature. The crystals were grown using NaCl–KCl flux at 1350 °C and B 2O 3 flux at 1150 °C. High growth temperature of 1350 °C for the NaCl–KCl flux caused Pt contamination from the crucible and oxygen deficiency for the crystals obtained. The crystal growth using B 2O 3 flux proceeded at lower temperature by grain growth with material transfer through B 2O 3. The crystal obtained was characterized by single-crystal XRD method, and was found to have a stoichiometric composition, Tl 2Ru 2O 7− δ ( δ=0), with a structural phase transition around 120 K. The grain growth technique with B 2O 3 is efficient for high-temperature single-crystal growth under high pressure.

Microstructure and microwave dielectric properties of BaLa 4Ti 4O 15 ceramics with template particles

BaLa 4Ti 4O 15 (BLT) ceramic has excellent microwave dielectric properties with high dielectric constant ( ɛ r) and high quality factor ( Q × f). However, BLT has the negative temperature dependence of resonant frequency ( τ f). In this study, single phase and plate-like BLT particles were prepared via a molten salt synthesis using NaCl, KCl and NaCl–KCl flux. Plate-like BLT template particles mixed with powders synthesized by solid-state reaction were uniaxially pressed and sintered. Sintered ceramics with template particles showed anisotropy in the microstructure and the crystalline phase. In the microwave dielectric properties, τ f and ɛ r increased with an increasing amount of template particles, and Q × f were nearly equal to that of specimens without template particles.

Molten salt synthesis of 0.94(Na1 / 2Bi1 / 2)TiO3–0.06BaTiO3 powder

High quality ceramics require pure and agglomerate-free powders. In the present study, 0.94(Na1/2Bi1/2)TiO3–0.06BaTiO3 (BNBT6) powders were prepared by molten salt synthesis (MSS) at 850 to 1100 °C. In the NaCl–KCl flux, a pure perovskite-phase BNBT6 powder was obtained at 850 °C with a grain size of below 100 nm, but the particles were highly aggregated. At 1100 °C, faceted single-crystalline particles were created with a size distribution around 1–3 μm, which were very easily dispersed and agglomerate-free. The phase and composition of synthesized particles were also studied. With increasing synthesis temperature, the BNBT6 powders showed a trend from the (Na0.5Bi0.5)TiO3 feature of rhomboidal symmetry to a typical BNBT6 that featured both rhombohedral and tetragonal symmetry.

Synthesis of layer-structured ferroelectric Bi 3NbTiO 9 plate-like seed crystals

Pure bismuth layer-structured ferroelectric Bi 3NbTiO 9 plate-like seed crystals are synthesized by molten-salt method in NaCl–KCl flux using Bi 3NbTiO 9 precursor from precipitation. Single phase Bi 3NbTiO 9 powder is obtained in NaCl–KCl flux at 700 °C for only 0.5 h, which is 200 °C lower than the formation temperature of pure Bi 3NbTiO 9 Aurivillius phase without chloride salts. The Bi 3NbTiO 9 platelet synthesized at 900 °C is well-dispersed with the diameter of 5–10 μm and the aspect ratio of 25–50.

The Praseodymium Zinc Arsenide Pr3Zn2As6: Crystallizing with a Vacancy Variant of the HfCuSi2Type Structure

The new compound Pr3Zn2As6has been prepared by reaction of the elemental components in a NaCl–KCl flux. It crystallizes with a very pronounced subcell of the HfCuSi2type structure with two formula units PrZn2/3As2in the tetragonal subcell witha=400.6(1) pm andc=1019.7(4) pm. The ordered distribution of occupied and unoccupied zinc positions leads to a three times larger cell. This structure was determined from four-circle X-ray diffractometer data of a twinned crystal:Pmmn,a=399.89(9) pm,b=1204.2(3) pm,c=1019.7(4) pm,Z=2,R=0.039 for 2663 structure factors and 39 variable parameters. Chemical bonding in this compound is briefly discussed.