Che-Yuan Yang

Microstructural examination of 2.25Cr1Mo Steel Steam pipes after extended service

Abstract Microstructural investigation has been carried out for 2.25Cr1Mo steel taken from service-exposed steam pipes. Samples selected for study experienced a nominal temperature of 542°C, with a nominal internal pressure 17.4MNm−2 for 5 and 18 years. The original microstructure of the pipes consisted of ferrite and pearlite. The service exposure caused carbide transformations and ferrite grain growth. In this work, color micrography taken from specimens examined in the light microscope, transmission electron microscopy, scanning transmission electron microscopy and energy-dispersive x-ray analysis on carbon extraction replicas and thin foils were utilized to understand the carbide distribution, morphology, and structure. A sequence for microstructural degradation is also proposed, and this provides a basis for understanding the behavior of steam pipes during extended service.

Structural and luminescence properties of tunable white-emitting Sr0.5Ca0.5Al2O4:Eu2+, Dy3+ for UV-excited white-LEDs

Tunable white-emitting Sr0.5Ca0.5Al2O4:Eu2+, Dy3+ phosphors were synthesized via solid-state reactions and evaluated as suitable candidates for white light emitting diodes in this study. The crystal structure of Sr0.5Ca0.5Al2O4:Eu2+, Dy3+ was refined and determined from X-ray diffraction (XRD) profiles via the Rietveld refinement method. The as-prepared phosphors exhibit a monoclinic structure with the P121/n1 space group. Under ultraviolet (UV) excitation, these phosphors display two emission bands, which peak at around 445 and 540 nm, corresponding to the 5d → 4f transition of Eu2+ ions. The photoluminescence results indicate that the Eu2+ ions tend to occupy three different crystallographic sites of the Ca2+ and Sr2+ ions in the Sr0.5Ca0.5Al2O4 structure. By the increment of Eu2+ ion concentration in Sr0.5Ca0.5Al2O4:Eu2+, Dy3+, the emission colors can be tuned from bluish white to warm white and eventually to yellowish white. Such color tuning has been achieved due to the resonance type energy transfer among the Eu2+ ions located at various crystallographic sites. The non-radiative energy transfer between Eu2+ ions is attributed to dipole–dipole interactions. The critical distances of the energy transfer are calculated to be 2.98 and 2.86 nm using concentration quenching and spectral overlap methods, respectively. The present research suggests that tunable white-emitting Sr0.5Ca0.5Al2O4:Eu2+, Dy3+ phosphors have the potentiality to be applied for UV excited white LEDs.

Structural evaluations and temperature dependent photoluminescence characterizations of Eu3+-activated SrZrO3 hollow spheres for luminescence thermometry applications

This research is focused on the temperature sensing ability of perovskite SrZrO3:Eu3+ hollow spheres synthesized via the sol-gel method followed by heating. The Rietveld refinement indicated that the precursors annealed at 1100 °C were crystallized to form orthorhombic SrZrO3. SrZrO3 particles exhibited non-agglomerated hollow spherical morphology with an average particle size of 300 nm. The UV-excited photoluminescence spectrum of SrZrO3:Eu3+ consisted of two regions. One region was associated with SrZrO3 trap emission, and the other one was related to the emission of Eu3+ ions. The intensity ratio of the emission of Eu3+ ions to the host emission (FIR) and the emission lifetime of Eu3+ ions were measured in the temperature range of 300–550 K. The sensitivity obtained via the lifetime method was 7.3× lower than that measured via the FIR. Within the optimum temperature range of 300–460 K, the as-estimated sensor sensitivity was increased from 0.0013 to 0.028 K−1. With a further increase in temperatures, the sensitivity started to decline. A maximum relative sensitivity was estimated to be 2.22%K−1 at 460 K. The resolutions in both methods were below 1K in the above temperature range. The results indicated the suitability of SrZrO3:Eu3+ for the distinct high temperature sensing applications.

Enhanced upconversion of NaYF₄:Er³⁺/Yb³⁺ phosphors prepared via the rapid microwave-assisted hydrothermal route at low temperature: phase and morphology control.

Monophasic NaYF4:Er(3+)/Yb(3+) crystals were synthesized via the microwave-assisted hydrothermal route at 180°C. Microwave heating during the hydrothermal process substantially reduces the duration of reaction for the formation of cubic-NaYF4:Er(3+)/Yb(3+) nanocrystals from 6 h to 30 min. As the duration of the reaction increases, cubic-NaYF4:Er(3+)/Yb(3+) nanocrystals are transformed to uniform hexagonal-NaYF4:Er(3+)/Yb(3+) microprisms because of the enhanced reaction kinetics. Bright upconverted emission from the NaYF4:Er(3+)/Yb(3+) crystal, obtained by the efficient two-photon excitation, is related to crystal structure and morphology. The hexagonal microprisms exhibit better upconversion and are employed in security applications.

Synthesis of Sr 2 Si 5 N 8 :Ce 3+ phosphors for white LEDs via efficient chemical vapor deposition

Novel chemical vapor deposition (CVD) process was successfully developed for the growth of Sr2Si5N8:Ce3+ phosphors with elevated luminescent properties. Metallic strontium was used as a vapor source for producing Sr3N2 vapor to react with Si3N4 powder via a homogeneous gas-solid reaction. The phosphors prepared via the CVD process showed high crystallinity, homogeneous particle size ranging from 8 to 10 μm, and high luminescence properties. In contrast, the phosphors prepared via the conventional solid-state reaction process exhibited relative low crystallinity, non-uniform particle size in the range of 0.5–5 μm and relatively lower luminescent properties than the phosphors synthesized via the CVD process. Upon the blue light excitation, Sr2−xCexSi5N8 phosphors exhibited a broad yellow band. A red shift of the emission band from 535 to 556 nm was observed with the increment in the doping amount of Ce3+ ions from x = 0.02 to x = 0.10. The maximum emission was observed at x = 0.06, and the external and internal quantum efficiencies were calculated to be 51% and 71%, respectively. Furthermore, the CVD derived optimum Sr1.94Ce0.06Si5N8 phosphor exhibited sufficient thermal stability for blue-LEDs and the activation energy was calculated to be 0.33 eV. The results demonstrate a potential synthesis process for nitride phosphors suitable for light emitting diodes.

Incorporation of selenium ions in solution-processed Cu(In,Ga)Se2 films

Cu(In,Ga)Se2 films were prepared on the molybdenum-coated soda-lime substrate using various processes to incorporate selenium ions. Selenium ion-containing solutions and selenium vapor were both used as the sources of selenium. When the precursors of Cu(In,Ga)Se2 were coated with selenium ion-containing solutions, the formation of MoSe2 was detected via secondary ion mass spectroscopy and GIXD analysis. The formation of thick MoSe2 films tended to retard the formation of Cu(In,Ga)Se2 and reduced the grain size of the prepared films. Therefore, the thick MoSe2 layer led to a decrease in the fill factor and the short-circuit density of the fabricated Cu(In,Ga)Se2 solar cells. The selenium ion-containing solutions were found to be easily evaporated during the heating process, thereby resulting in a decrease in the thickness and the porous microstructures of films. The decrease in the thickness of Cu(In,Ga)Se2 led to the decrease in the fill factors and the short-circuit density of the fabricated devices. The efficiency of Cu(In,Ga)Se2 solar cells prepared via selenium vapor was higher than that prepared via selenium ions. The photovoltaic characteristics of the fabricated solar cells were demonstrated to depend substantially on the routes of supplying and incorporation of selenium ions at elevated temperatures.

Corrigendum: Synthesis of Sr2Si5N8: Ce3+ phosphors for white LEDs via efficient chemical vapor deposition

This corrects the article DOI: 10.1038/srep45832.

Synthesis and characterization of high concentration Nd3+ doped YAG nanopowders for laser applications

Herein, neodymium-doped yttrium aluminum garnet (Nd:YAG) ceramic powders were fabricated via a wet chemical reaction method followed by heating in air. The structural studied confirmed the formation of single phase Nd:YAG nanocrystallites without any intermediate phase at 1000 °C, and the average particle size was calculated to be 260 nm. According to the reflectance results, the highest absoption of the synthesized powders was observed at 808 nm. Therefore, A laser diode (808 nm) with power output about 1000 mW was used as a pump source and the emission spectra was recorded. Upon IR excitation, the present sample showed intense emission at around 1064 nm and found to be potential for solid state lasewr devices.

Cu(In,Ga)Se 2 thin films codoped with sodium and bismuth ions for the use in the solar cells

The codoping effects of sodium and bismuth ions on the characteristics of Cu(In,Ga)Se2 films prepared via the solution process were investigated in this study. When sodium and bismuth ions were incorporated into Cu(In,Ga)Se2, the ratio of the intensity of (112) diffraction peak to that of (220/204) diffraction peak was greatly increased. The codoping process not only enlarged the sizes of the grains in the films but also resulted in densification of the films. The carrier concentration of Cu(In,Ga)Se2 was found to be effectively increased to cause a reduction in the resistivity of the films. The above phenomena were attributed to the densified microstructures of the films and a decrease in the amount of the donor-type defects. The leakage current of the solar cells was found to be also decreased via the codoping process. Owing to the improved electrical properties of Cu(In,Ga)Se2 films, the conversion efficiency of the fabricated solar cells was significantly enhanced.

Controlled orientation and improved photovoltaic characteristics of Cu(In,Ga)Se 2 solar cells via using In 2 Se 3 seeding layers

In2Se3 films were utilized as seeding layers in the synthesis of Cu(In,Ga)Se2 films via the spin-coating route. Selenizing the indium-containing precursors at 400°C resulted in the formation of the hexagonal γ-In2Se3 with the preferred (006) orientation. Increasing the selenization temperature to 500°C yielded the (300)-oriented γ-In2Se3. Using the preferred (006)-oriented In2Se3 as seeding layers produced the preferred (112)-oriented Cu(In,Ga)Se2 film because of the crystalline symmetry. In contrast, the use of the (300)- oriented In2Se3 as seeding layers yielded the (220/204)-oriented Cu(In,Ga)Se2 films. According to results obtained using SEM and the Hall effect, (112)-oriented Cu(In,Ga)Se2 films had a denser morphology and more favorable electrical properties. Using the (112)-oriented Cu(In,Ga)Se2 films as the absorber layer in the solar devices resulted in a significant increase in the conversion efficiency.