Browse

1 - 10 of 1,021 items :

  • Materials Sciences, other x
  • Materials Characterization and Properties x
Clear All

Abstract

Usually, remote phosphor structures are beneficial in terms of flux but unfavorable in terms of color quality compared to conformal phosphor or in-cup phosphor packages. To eliminate this disadvantage, many studies have focused on increasing the chromatic quality of the remote phosphor configuration, which requires great efforts in improving two parameters of color: color rendering index (CRI) and color quality scale (CQS). CRI is known as the most useful quantitative method used to measure the ability of a light source to reproduce the colors of illuminated objects faithfully and naturally. Similarly, CQS is also a method of lighting quality determination and analysis, especially used as an alternative to the unsaturated CRI colors. In this paper, we proposed dual-layer remote phosphor structure as a novel method of CRI and CQS enhancement to improve WLEDs’ color quality. Five alike WLEDs but having different color temperatures in the range of 5600 K to 8500 K were applied in this study. The idea behind the study is to place a red phosphor layer SrwFxByOz:Eu2+,Sm2+ on the yellow phosphor layer YAG:Ce3+ and then determining an appropriate concentration of SrwFxByOz:Eu2+,Sm2+ added to achieve the highest color quality. The results point out that SrwFxByOz:Eu2+,Sm2+ brings great benefits to the improvement of CRI and CQS parameters. Specifically, the higher the SrwFxByOz:Eu2+,Sm2+ concentration results in the greater CRI and CQS, owning to the enriched red light components in the WLEDs. However, the flux has a tendency of dropping when SrwFxByOz:Eu2+,Sm2+ concentration rises excessively. This has been proved by using the Mie-scattering theory and the Lambert-Beer law. The results of this article are essential references for manufacturing WLEDs with higher chromatic quality.

Abstract

Good-quality semiorganic single crystals of bis(glycine) strontium dichloride trihydrate (GSC) were grown by slow evaporation technique. The lattice parameters of the grown crystal were verified through single crystal X-ray diffraction. Besides, the crystallinity of the material was analyzed with powder X-ray diffraction. The fingerprint of functional groups of this material was analyzed by FT-IR technique. The UV transparency cutoff wavelength of GSC was found to be 240 nm and the crystal exhibited 80 % transmission in the entire visible region. All intermolecular and intramolecular interactions of the grown crystal were interpreted by Hirshfeld surface analysis, and the strength of the interactions was graphically illustrated by fingerprint graphs. The intramolecular charge transfers of the crystal were assessed through frontier molecular orbital analysis.

Abstract

Cellulose nanocrystals were extracted from cotton. The cellulose nanocrystals made a self-assembly structure when dried under slow conditions, as it was revealed by the characterization made to the material. The AFM images of the nanocrystals showed that they had a changing local orientation, pointing in a preferred direction that underwent a periodic change. This periodic change resembles the orientation of a chiral nematic phase. The TEM images showed that the nanocrystals had a rod-like appearance with average length size of 98.5 nm and a diameter of 4.7 nm. The TEM characterization showed the nanocrystals with more details than AFM. In this paper, the self-assembling of CNC was observed by AFM, and further investigations were done by TEM, deconvoluting the process of CNC nanorods aggregation.

Abstract

This paper reports the synthesis and characterization of the Bi0:94Gd0:06FeO3 sample obtained by solid state reaction method. The structural and morphological analysis was performed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Rietveld refinement analysis, confirming the obtaining of a rhombohedral crystalline phase and R3c space group (a = b = 5.577 Å and c = 13.847 Å), with an interconnected structure. Studies by X-ray photoelectron spectroscopy (XPS) revealed trivalent oxidation states of Bi, Fe and Gd ions. The synthesized sample exhibited a non-linear M–H loop indicating a weak ferromagnetic behavior with a remnant magnetization of 6.34 emu˙mol−1 and a coercive field of 295 Oe. The obtained structural and magnetic characteristics make these materials of great interest as multiferroic components.

Abstract

The structural, electronic and optical properties of BexZn1−xO alloys were studied using the density functional theory and Hubbard-U method. Uo;p = 10.2 eV for O 2p and UZn;d = 1.4 eV for Zn 3d were adopted as the Hubbard U values. For BexZn1−x O alloys, the lattice constants a and c decrease linearly as Be concentration increases, the bandgap increases with a large bowing parameter of 6.95 eV, the formation enthalpies have the maximum value with Be concentration at 0.625, corresponding to the possible Be concentration to form phase separation. These calculations comply well with the experimental and other theoretical results. Furthermore, optical properties, such as dielectric function ∈(ω), reflectivity R(ω), absorption coefficient α(ω), were calculated and discussed for BexZn1−x O alloys with the incident photon energy ranging from 0 eV to 30 eV.

Abstract

Porous silica, silica-cobalt, silica-zirconia and zirconia membranes were synthesized by the sol-gel method. Multi-step coating (two, six, and ten steps) was used to reduce the defectiveness of the mesoporous layer. Scanning electron microscopy (SEM) images indicated that an increase in the number of coating steps improved the mesoporous layer quality. The results obtained from gas permeability tests with nitrogen and argon, however, indicated a reduction in the gas permeability with increasing coating steps. The reduction in gas permeability from two to six coating steps was more pronounced than from sixto ten- coating steps. It was found that six-step coating was economically justified in obtaining a uniform mesoporous layer. The results of pore radius calculations by Knudsen flow mechanism revealed that the pores in the silica, silica-cobalt, and zirconia membranes were in the mesoporous range. The sols with a mean particle size more than 100 nm are not recommended for synthesis of mesoporous layer free of defects. Furthermore, the type of acid used as a catalyst is also important in obtaining a layer without defectiveness.

Abstract

Co@Co3O4@Nitrogen doped carbon (Co@Co3O4@NDC) composite is synthesized by high temperature carbonization of ionic liquids followed by low temperature thermal oxidation. In the process of high temperature carbonization, cobalt ions are reduced to metallic cobalt, producing Co@Nitrogen doped carbon (Co@NDC). Co@Co3O4@NDC composite is obtained after low temperature oxidation, in which a part of the metallic cobalt is oxidized to Co3O4. The structural characterizations indicate that the composite is composed of three crystalline phases (carbon, Co and Co3O4). The results of transmission electron microscopy study show that the carbon materials not only coat the Co@Co3O4 nanoparticles, but also form carbon network that connects the Co@Co3O4 nanoparticles. This conductive carbon network is beneficial to improve the electrochemical performance of the composite. The electrochemical test results show that the Co@Co3O4@NDC composite exhibits excellent electrochemical performance, delivering the discharge capacities of 790 and 304 mAh·g−1 after 1500 cycles at 5 C and 10 C. This excellent electrochemical performance is due to synergistic effects of Co3O4, cobalt nanoparticles embedded in carbon which has high conductivity, and nitrogen functional groups.

Abstract

Hybrid white light-emitting devices (HWLEDs) were fabricated using FTO/PEDOT: PSS/PbS/Alq3/Ni system and synthesized by phase separation process. In the present study, the multiple excitons generation in lead sulfide (PbS) NCs, which is characteristic of PbS NCs, was used to induce an effective and regulated energy transfer to an HWLED. The HWLED consisted of three layers successively deposited on FTO glass substrate; the first layer consisted of poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) blended with polymethyl methacrylate (PMMA) organic polymer in the 1:1 ratio, while the second layer consisted of PbS NCs. Finally, above the layer of the PbS NCs, Tris (8-hydroxyquinoline) aluminum (Alq3) layer was deposited. The white light was generated with quite a good efficiency due to the confinement effect that makes the energy gap greater. The characteristics of the current-voltage (I-V) indicate acceptable conditions for the generation of white light by multiple excitons. It was found that the emission levels able to produce white luminescence, classified based on the coordinate system of chromaticity (CIE 1931), are x = 0.31, y = 0.33 while the correlated color temperature (CCT) is about 6250 K. The HWLEDs made from PbS NCs with hole injection from the organic polymer (PEDOT: PSS with PMMA), and electron injection from organic molecules (Alq3) are capable of white light generation.

Abstract

A sequence of N-doped carbon materials has been synthesized using poly(acrylonitrile)-ionic liquid copolymers as carbon precursors. The nitrogen content and configuration in carbon materials has been changed regularly within a certain range by adjusting the proportion of ionic liquids. We found that the capacity and rate performance increased dramatically after the introduction of ionic liquids, which was attributed to incorporation of higher amount pyridinic-N, pyrrolic-N into the carbon materials. Besides, with the increase of the graphitic-N, the initial Coulombic efficiency decreased from 58.5 % to 53.47 % and the RSEI raised from 66.34 Ω to 140.96 Ω, which was attributed to the higher cohesive energy of Li dimmer than adsorption energy of graphitic-N with Li, since more lithium clusters during the formation of SEI film were formed. The electrochemical tests also revealed the negative role of graphitic-N in the capacity. Therefore, this work provides a feasible method to design the nitrogen content and configuration of the N-doped carbon materials.

Abstract

Laser-induced local crystallization in Finemet-type alloy was studied using X-ray diffraction, SEM and EDX methods. For investigated conditions of irradiation (wavelength λ = 1.06 µm, laser power density 50 W/cm2), it was found that primary crystallization starts with the formation of the nanocrystalline α-Fe(Si) solid solution at shorter exposure time and the second step crystallization with the nanocrystalline hexagonal H-phase formation occurs in longer exposure time. Changes in the local element concentration were observed at the surface of the irradiated zone and at the ribbon fracture. It was shown that the nonlinear temperature field due to the laser irradiation resulted in changes of the local elements concentration and this feature changed crystallization mechanism of the Finemet-type alloy.