Crystallization Of Amorphous Precursors Research Articles

Features of the Phase Preferences, Long- and Short-Range Order in Ln2(WO4)3 (Ln = Gd, Dy, Ho, Yb) with Their Relation to Hydration Behavior

The effect of synthesis conditions on the features of the long- and short-range order of Ln2(WO4)3 (Ln = Gd, Dy, Ho, Yb) powders synthesized via coprecipitation of salts has been studied by a complex of physico-chemical techniques including synchrotron X-ray powder diffraction, X-ray absorption spectroscopy, Raman and infrared spectroscopy, and simultaneous thermal analysis. It was found that crystallization of amorphous precursors begins at 600 °C/3 h and leads to the formation of the monoclinic structure with sp. gr. C12/c1(15) for Ln2(WO4)3 (Ln = Gd, Dy) and with sp. gr. P121/a1(14) for Ln = Yb, whereas crystallization of Ho precursor requires even higher temperature. After annealing at 1000 °C, the P121/a1(14) phase becomes the dominant phase component for all heavy lanthanoid types except for Ln = Gd. It was shown that the Ln (Ln = Dy, Ho, and Yb) tungstates with the P121/a1(14) monoclinic structure correspond to trihydrates Ln2(WO4)3·3H2O formed due to a rapid spontaneous hydration under ambient conditions. It was concluded that the proneness to hydration is due to a specific structure of the P121/a1(14) phase with large voids available to water molecules. Modifications in the local structure of Ln-O coordination shell accompanying the structure type change and hydration are monitored using EXAFS spectroscopy.

In situ decoration of SnS quantum dots on the α-SnWO4 nanosheets for superior visible-light photocatalytic performance

A novel heterostructure consisting of SnS quantum dots (QDs) and α-SnWO4 nanosheets was constructed through the crystallization of amorphous precursor using a one-pot hydrothermal method. The formation of SnS/α-SnWO4 heterostructure was confirmed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) techniques: the SnS QDs with the average particle size of 2.2 nm were uniformly anchored on the surface of α-SnWO4 nanosheets. The amorphous α-SnWO4 precursor with negative charge can attract the excessive Sn2+ ions and further react with S2- to form SnS/α-SnWO4 heterojunction. The intimate contact between SnS QDs and α-SnWO4 nanosheets was beneficial to the effective separation of photo-created electron-hole pairs. Thus, the SnS/α-SnWO4 nanocomposites exhibited the enhanced photocatalytic activity in the degradation of methyl orange (MO), Rhodamine B (RhB) and tetracycline (TC). The optimal amount of SnS QDs on the α-SnWO4 surface was 2 mol%, whose photodegradation efficiency was about 95.56% for MO removal after 90 min visible-light illumination and its degradation rate (0.0338 min−1) was 8.7 times higher than that of pure α-SnWO4 (0.0039 min−1). The trapping experiments evidenced that the ·O2–, h+ and ·OH play important roles in the photocatalysis process. The superior photocatalytic performance of SnS/α-SnWO4 nanocomposite would be ascribed to the Z-scheme photocatalytic reaction mechanism.

Graphene/MxWO3 (M = Na, K) nanohybrids with excellent electrical properties

Graphene/MxWO3 (M=Na, K) nanocomposites with different weight ratios were successfully synthesized using MBH4 as a reducing agent in aqueous solution, followed by crystallization of amorphous precursors under calcination in H2 (5vol.%)/N2 atmosphere at 600°C. The coupled samples showed good shielding properties in NIR wavelength range together with certain visible lights transparency. The as-prepared tungsten bronze nanoparticles are homogeneously anchored on graphene sheets, and the graphene/tungsten bronze nanocomposite showed an enhanced electrical conductivity with good shielding properties in the NIR range together with certain visible lights transparency. With the increment of graphene amount in the composites, the contact between the graphene sheets was enhanced, and the MxWO3 might enhance the charge transport properties of graphene by generating the short conductive path for electrons over the remained oxygenated functional groups. The synergistic effects could be observed in the composites. When 5wt.% or more graphene were composed into MxWO3, the hybrid composite showed the higher electrical conductivity than those of uncoupled graphene and tungsten bronze alone.

Solvothermal synthesis and electrical properties of rGO/MxWO3 (M=Na, K) nanocomposites

MxWO3 (M=Na, K) nanoparticles were successfully synthesized using MBH4 as a reducing agent in aqueous solution, followed by crystallization of amorphous precursors under calcination in H2 (5 vol.%)/N2 atmosphere at 600 °C. Then, it was post-graphenized by solvothermal method with different weight ratios. The synergistic effects could be observed in the composites. When 10 wt.% or more graphene were composed into MxWO3, the hybrid composite showed the higher electrical conductivity than those of uncoupled graphene and tungsten bronze alone.

New Reactor Fabricated Using Light Leakage Fiber for Azo Dye Degradation

Lanthanum-doped anatase TiO2coatings, which are composed of assemble crystalline of 50 nm diameter particles, when the percentage of dopant is 0.5 wt%, have been successfully fabricated by solgel dip-coating process on light leakage silica fiber (LSF) which length is 15cm and diameter is 125μm. This was achieved by adjustment of the lanthanum-doped solgel parameters such as molar ratio of precursors in lanthanum-doped TiO2-sols, the ratio of titanium tetrabutoxide to polyvinyl alcohol, dip-coating velocity, drying duration in air, thermal treatment and number of cyclical time of the process. Titania nanocrystals were prepared at ambient temperature in a liquid media using titanium tetrabutoxide as precursor, and the crystallization of amorphous precursor was preceded by peptizing with acid and then refluxing for a periodic time in homothermal water-bath. The photocatalytic properties of the La-TiO2films had been testified by the photo degradation of methyl orange. The lanthanum-doped anatase TiO2thin films were characterized by XRD and TEM. The products show much improved photocatalytic activity that will be useful in the design of a novel antibacterial, deodorant and antipollution new photo reactor.

Fabrication of Nb-Doped TiO2Transparent Conducting Films by Postdeposition Annealing under Nitrogen Atmosphere

Here, we report that highly conductive polycrystalline anatase Nb-doped TiO2 (TNO) thin films can be prepared via crystallization of amorphous precursors under N2 atmosphere. An optimized TNO film on a glass substrate exhibited a low resistivity of 8.4 ×10-4 Ω cm and an absorbance of 6% at a wavelength of 460 nm. These transport and optical properties were comparable to those of TNO films fabricated by vacuum annealing. This demonstrates the potential of TNO as an electrode for GaN-based light-emitting diodes.

Fabrication of Nb-Doped TiO2 Transparent Conducting Films by Postdeposition Annealing under Nitrogen Atmosphere

Here, we report that highly conductive polycrystalline anatase Nb-doped TiO2 (TNO) thin films can be prepared via crystallization of amorphous precursors under N2 atmosphere. An optimized TNO film on a glass substrate exhibited a low resistivity of 8.4 ×10-4 Ω cm and an absorbance of 6% at a wavelength of 460 nm. These transport and optical properties were comparable to those of TNO films fabricated by vacuum annealing. This demonstrates the potential of TNO as an electrode for GaN-based light-emitting diodes.

Lanthanum-Doped Titania Film Coated on Light Leakage Fiber Photo-Degradation Methyl Orange

Lanthanum-doped anatase TiO2 coatings, which are composed of assemble crystalline of 50 nm diameter particles, when the percentage of dopant is 0.5 wt%, have been successfully fabricated by sol–gel dip-coating process on light leakage silica fiber (LSF) which length is 15cm and diameter is 125μm. This was achieved by adjustment of the lanthanum-doped sol–gel parameters such as molar ratio of precursors in lanthanum-doped TiO2-sols, the ratio of titanium tetrabutoxide to polyvinyl alcohol, dip-coating velocity, drying duration in air, thermal treatment and number of cyclical time of the process. Titania nano crystals were prepared at ambient temperature in a liquid media using titanium tetrabutoxide as precursor, and the crystallization of amorphous precursor was preceded by peptizing with acid and then refluxing for a periodic time in homothermal water-bath. The photocatalytic properties of the La-TiO2 films had been testified by the photo degradation of methyl orange. The lanthanum-doped anatase TiO2 thin films were characterized by XRD and TEM. The products show much improved photocatalytic activity that will be useful in the design of a novel antibacterial, deodorant and antipollution photo reactor.

Effects of Crystallization Atmospheres on the Near‐Infrared Absorbtion and Electroconductive Properties of Tungsten Bronze Type MxWO3 (M = Na,K)

The fine particles of tetragonal KxWO3 and cubic NaxWO3 were successfully synthesized by reducing WO42− with BH4− in aqueous solutions, followed by the crystallization of amorphous precursors by calcination under N2, NH3 and H2 (5 vol%)/N2 atmospheres at 600°C for 1 h, respectively. The products showed excellent electrical conductivity, visible light transparency, and near‐infrared light shielding characteristics, where the samples crystallized in H2 (5 vol%)/N2 mixed gas showed the highest electric conductivity being superior to commercial conductive oxide ITO. Meanwhile, the samples showed strong absorption of near‐infrared light and instantaneous conversion of photo‐energy to local heat. The effects of crystallization atmosphere on the near‐infrared absorbtion and electroconductivity properties of KxWO3 and NaxWO3 are discussed in detail.

Preparation and Characterization of Lanthanum (La)-Doped TiO<sub>2</sub>

Lanthanum-doped anatase TiO2 coatings, which were composed of assemble crystalline of 50 nm diameter particles have been successfully fabricated by sol–gel dip-coating process on light leakage quartz fiber (LQF) which length is 15cm and diameter is 125μm. This was achieved by adjustment of the lanthanum-doped sol–gel parameters such as molar ratio of precursors in lanthanum-doped TiO2-sols, the ratio of titanium tetrabutoxide to polyvinyl alcohol, dip-coating velocity, drying duration in air, thermal treatment and number of cyclical time of the process. Titania nano crystals were prepared by ambient temperature in a liquid media using titanium tetrabutoxide as precursor, and the crystallization of amorphous precursor was preceded by peptizing with acid and then refluxing for a periodic time in homothermal water-bath. The photocatalytic properties of the lanthanum-doped TiO2 films were testified by the photo degradation of methyl orange. The lanthanum-doped anatase TiO2 thin films were characterized by XRD and TEM.

Effect of Cr on the magnetic properties and microstructure of Nd2Fe14B/α-Fe nanocomposites

Abstract The magnetic properties of Nd 11 Fe 72− x Co 8 V 1.5 Cr x B 7.5 ( x =0,1) nanocomposites prepared by the crystallization of amorphous precursors have been investigated. It was found that Cr addition increases the crystallization temperature of the amorphous ribbons, resulting in the formation of a metastable phase of Nd 2 Fe 23 B 3 and improved magnetic properties. After optimal thermal treatment, the intrinsic coercivity ( J H c ) and maximum magnetic energy product (( BH ) max ) increase from 780 kA/m, 68 kJ/m 3 for Nd 11 Fe 72 Co 8 V 1.5 B 7.5 sample to 903.5 kA/m, 71 kJ/m 3 for Nd 11 Fe 71 Co 8 V 1.5 Cr 1 B 7.5 sample, respectively.

Effect of Mn on the magnetic properties of Fe 3B/Nd 2Fe 14B nanocomposites

The magnetic properties of Nd 4.5Fe 77− x Mn x B 18.5 ( x=0, 1 and 2) nanocomposites prepared by the crystallization of amorphous precursors were investigated. Addition of Mn is found to decrease the crystallization temperature of the amorphous ribbons. The intrinsic coercivity i H c and maximum energy product ( BH) max increase from 2.6 kOe and 9.1 MGOe for x=0 to 3.1 kOe and 10.3 MGOe for x=1, respectively, and the remanence ratio M r/ M s increases from 0.70 to 0.72. The effect of Mn on Curie temperature T C and the thermal stability of M r and i H c were also studied. 57Fe Mössbauer spectra have been recorded for x=0, 1 and 2 ribbons at room temperature and site preference of the Mn atoms in Fe 3B and Nd 2Fe 14B phases is discussed using the Mössbauer spectroscopy.

Photocatalysis anatase thin film coated PAN fibers prepared at low temperature

In order to improve the repeating circles of the semiconductor titania photocatalyst and broaden its applications, thin film coatings of titania based on polyacrylonitrile fibers (TiO 2/PAN) were prepared successfully at room temperature by a simple dip-coating method. Titania nanocrystals were prepared by a low temperature dissolution–reprecipitation process in a liquid media using TiCl 4 as precursor, and the crystallization of amorphous precursor was proceeded by peptizing with acid and then refluxing for a periodic time. The films had been characterized by XRD, UV–vis, SEM and the photocatalytical properties of the titania films had been testified by the photodegradation of methylene blue (MB). SEM images showed the information of continuous layers of titania on PAN fibers, XRD patterns revealed the main phase of the titania particles and titania coatings were anatase. UV–vis analyses revealed the coated titania produced an excellent UV protection of the coated fiber. The fibers with anatase coatings showed high photocatalytic property and better repetition on the photodegradation of MB. The titania coated fibers are expected to be applied as a novel antibacterial, deodoring and antifouling photocatalyst.

Crystallization of amorphous FeNiZrB and FeNiCoZrB alloys

Crystallization of amorphous precursors: Fe31Ni50Zr7B12, Fe31Ni40Co10Zr7B12 and Fe31Ni30Co20Zr7B12 was studied by the Mossbauer spectroscopy and X-ray diffraction. The nanocrystalline phases were formed by annealing the amorphous alloys at temperatures of 440–770°C. The crystallization temperatures were determined by DSC measurements. Addition of Co atoms into FeNiZrB alloy resulted in the formation of FeCo phase with large hyperfine field of 35 T. Formation of FeCo and (FeNiCo)23B6 phases was followed by the Mossbauer measurements and confirmed by XRD measurements. The size of nanograins was estimated from XRD measurements using the Scherrer formula.

Nanogranular Phase Formation During Devitrification of Fe(NiCo)ZrB Amorphous Alloys

A comprehensive review of our recent experimental and theoretical developments in the processing of nanocrystalline soft magnetic materials made by crystallization of amorphous precursors and containing new nanocrystalline phases is given. The relationship between the structures of the metastable and equilibrium phases and their transformations are discussed. Nickel-rich amorphous precursors with stoichiometry Ni64Fe16Zr7B12Au1 were produced by melt-spinning technique and then heat-treated at temperatures ranged from 420 °C to 600 °C for one hour to form nanostructured alloy. The transformation from the amorphous state into the nanocrystalline state was investigated by the differential scanning calorimetry (DSC), the x-ray diffraction (XRD), the vibrating sample magnetometry (VSM) and Mossbauer techniques. The annealing favours the emergence of cubic FexNi23-xB6 crystalline grains (10-25 nm in diameter). Magnetic measurements made at 4.2-1100 K reveal rather high value of saturation magnetization (nearly 60 and 40 Am2/kg at 4.2 K and room temperature, respectively) in amorphous as well as in nanocrystalline states. These facts are consistent with 300 K 57Fe Mossbauer results which are well supported by the calculations of Ni and Fe magnetic moments in Ni23B6 and Fe23B6 phases, using the spin polarized tight binding linear muffin-tin orbital (TB-LMTO) method. However, anomalous high magnetic moments of Fe and Co atoms were found in some inequivalent positions in the crystal structure.

Mg–Ni–RE nanocrystalline alloys for hydrogen storage

Various nanocrystalline and nano-amorphous Mg–Ni–RE (RE=rare earth or Y) alloys with attractive hydrogen storage properties were prepared by direct quenching of the melt and by crystallization of amorphous precursors. Varying the quenching rate, different microstructures could be obtained by melt-spinning. Having a knowledge of the mechanism and kinetics of the crystallization process, the devitrification of the amorphous samples could be carried out in a controlled way leading to nanocrystalline material. The hydrogenation kinetics and the H-storage capacity of amorphous, partially and completely nanocrystalline alloys were investigated and it was found that they are dependent on the microstructure and on the phase composition of the alloys. Therefore, the crystallization of the amorphous or nano-amorphous alloys was thoroughly studied, applying different experimental methods and approaches. The sequence of the crystal phases formation and the development of the alloys microstructure were investigated. Crystallization kinetics and mechanisms were determined as well.

Synthesis of visible-light-active nanosize rutile titania photocatalyst by low temperature dissolution–reprecipitation process

Titania nanocrystals were prepared by a “low temperature dissolution–reprecipitation process” (LTDRP) in liquid media. The crystallization of amorphous precursor could proceed at low temperature around room temperature, which was much lower than those of conventional calcination and hydrothermal reactions. The thermodynamically stable rutile formed at low temperature below 70°C, while the metastable anatase formed at higher temperature. The phase composition, microstructure, morphology, and specific surface area of titania changed significantly depending on the reprecipitation temperatures. Needle-like rutile titania and spherical anatase titania crystals with high specific surface areas were prepared. Well-crystallized needle-like nanosize rutile crystals possessed higher photocatalytic activities than those of anatase crystals under visible light irradiation of wavelength >400 and/or >510nm.

Transport and magnetic properties of HITPERM alloys

Nanocrystalline HITPERM alloys Fe44.6Co43.3X7.4B3.7Cu1 (X = Nb,Zr, Hf) prepared by crystallization of amorphous precursors are studiedby magnetization and electrical resistivity measurements for the firsttime. Structural and magnetic components of the electrical resistivityare separated. The electrical resistivity of the nanocrystalline α′(FeCo) phase calculated using the Maxwell–Garnett relation proves strongelectron scattering on the grain boundaries. The temperature variation of thecrystalline fraction during the first crystallization stage is calculated for the Hfbased alloy.

Early Stages of Mechanical Crystallization of Amorphous Fe86Zr7B6Cu1 Soft Magnetic Material: The Role of Shear Bands

Nanocrystalline soft magnetic materials can have superior properties such as low coercivity combined with a high saturation magnetization. Whereas these materials are usually made by thermal crystallization of amorphous precursors, mechanical crystallization using ball milling provides an alternative route for the generation of nanocrystalline materials. The focus of this study is the development of the magnetic properties and the microstructure at the early stages of the transformation of initially amorphous Fe 86 Zr 7 B 6 Cu 1 . Controlled deformation of amorphous starting material by cold rolling is performed to clarify the origin of the coercivity maximum. Optical and magneto-optical investigations reveal the influence of the microstructure on the coercivity. The shear band density obtained from the magneto-optical observations is directly correlated with the increase in coercivity.

Low temperature synthesis of nanosize rutile titania crystal in liquid media

Titania powders consisting of well-crystallized nanosize rutile phase (ca. 20–50 nm) were prepared by the “low temperature dissolution–reprecipitation process” (LTDRP) in liquid media. The crystallization of amorphous precursor was carried out at a low temperature around room temperature. The crystallization temperature was about 385 and 110 °C lower than those by calcination in air (407 °C) and hydrothermal reaction (130 °C), respectively. The phase constitution of powders from LTDRP was identified by XRD and Raman analyses. The titania powders prepared by LTDRP possessed a high specific surface area of 135.0 m 2 g −1, which was about two times greater than that prepared by hydrothermal method. The phase composition, microstructure, morphology, agglomerate state and specific surface area of titania changed significantly, depending on the pH value, type of solvent and coexisting ion, etc. in the solution. The photocatalytic activity of crystalline titania was characterized.