In-Gann Chen

Enhancement of white light emission from novel Ca3Y2Si3O12:Dy3+ phosphors with Ce3+ ion codoping

The luminescent properties of the Ce3+ or Dy3+ singly doped and Ce3+/Dy3+ codoped in Ca3Y2Si3O12 novel phosphors were investigated, which are prepared by a sol-gel method. Ce3+ doped phosphor showed a brighter and broader violet-blue color emission band with a maximum peak centered at 389 nm, which is attributed to the parity and spin allowed 5d–4f transition. Photoluminescence spectra reveals that the white color emission is originated from the mixtures of two characteristic emission bands of Dy3+ ion, viz., the 473 nm blue emission (F49/2-H615/2) and the 580 nm yellow emission (F49/2-H613/2). codoping of Ce3+ has enhanced the luminescence of Dy3+ quite significantly upon the UV excitation wavelength (242 nm) and the optimized codopant concentration of Ce3+ is found to be 3 mol %. The mechanism involved in the energy transfer between Ce3+ and Dy3+ has been elucidated by an energy level diagram. The structure and morphology of the prepared samples have been analyzed by x-ray diffraction and transmission e...

Preparation and magnetic properties of Ba–Co2Z and Sr–Zn2Y ferrites

We have prepared Ba–Co2Z and Sr–Zn2Y ferrites by mixing SrCO3 (BaCO3), Co2O3 (ZnO), and Fe2O3 and calcinating in air and O2. The effects of sintering atmosphere (O2 and Ar) and sintering temperatures (1373–1573 K) on the structural, magnetic, and electrical properties have been measured. The Ba–Co2Z sample sintered at 1573 K under a flowing O2 exhibits superior dc electrical resistivity (∼108 Ω-cm), high initial permeability (u′∼21), and low loss factor (F∼1.6×10−3) at 20 MHz. The related XRD and SEM/EDS analyses of these samples have also been studied to correlate these properties. These results indicate that significant modification of magnetic properties up to 20 MHz can be achieved with the addition of the quaternary component and proper control of the processing oxygen atmosphere.

The influence of post-growth thermal treatments on the critical current density of TSMG YBCO bulk superconductors

Oxygenation and thermochemical post-growth treatments of top seeded melt-growth (TSMG) YBCO bulk superconductors can significantly influence critical current density. It is shown that, depending on oxygenation conditions and the size of 211 particles, different reductions of intrinsic critical current density values can be obtained due to the reduction in the sample cross-section caused by the presence of a/b-microcracks induced by 211 particles, and a/b-?and a/c-cracks induced by oxygenation. The possibility of eliminating oxygenation cracks by high pressure oxygenation and consequently significantly increasing the macroscopic critical current density is demonstrated. An effective dopant concentration for chemical pinning is proposed and possible clustering of substitutions in the Y123 lattice by thermochemical treatments is shown.

Size effect of Y211 additions in Sm–Ba–Cu–O materials

Our previous studies indicate that small additions of nano-scale RE2BaCuO5 (RE211, RE: rare elements) form nano-sized compositional fluctuations during the melt-growth process and thus act as field induced pinning centres. The solubility, peritectic temperature, and atomic size of these nmRE211 additives alter the Jc–H performance. To further reveal the effect of size and amount of RE211 additives, in this study, Y2Ba1Cu1O5 (Y211) particles of two different sizes, micro-scale (µm) and nano-scale (nm), were used. The superconducting results indicate that the contribution of Jc in high field regions is larger in nmY211 than µmY 211. The microstructure results imply that µmY 211 particles tend to form Y-rich regions in the core shell, while nmY211 particles tend to produce regions with Y fluctuations and high defect density. These different results are attributed to the reactions between the Y211 additions and the 123 matrix during the melt-textured growth process.

Enhancement of the superconducting properties of air-processed melt-growth Sm–Ba–Cu–O with the addition of Sm2BaO4

In this paper we reveal a systematic study of a series of samples with various Sm2BaO4 additions to compare the effect of the initial composition on the superconductivity. It is found that with the addition of 40 mol% Sm2BaO4 (Sm210) a significant improvement of superconductivity (Tc ~ 89 K, Jc(77 K, 1 T) ~ 104 A cm−2) can be achieved with melted-textured growth under ambient conditions. Both the Sm2BaCuO5 (Sm211) particle size and the Sm/Ba ratio in the SmBa2Cu3O7 (Sm123) matrix are reduced. We discuss in detail the microstructural evolution, the Sm/Ba substitution ratio and the superconducting properties.

Improvement to superconductivity using small amounts of Pd addition in Sm-Ba-Cu-O materials

We have studied the superconducting properties of air-processed melt-textured grown Sm–Ba–Cu–O samples with the addition of small amounts of Pd (0–1.0 wt%). The superconducting properties are enhanced with a small amount (0.1 wt%) of Pd addition, for which Tc is ~89 K and Jc at 0 T and 77 K is 104 A cm−2. However, both Tc and Jc deteriorate as the Pd addition is further increased. In addition, Jc of Pd-doped samples exhibits anisotropic Jc(H, T) behaviour. Observation of the microstructures has shown that, as the Pd addition increases, the shape of the Sm2BaCuO5 particles (211-particles) tends to become rectangular, and the 211-particle size increases (larger than the control sample). We have observed changes in size and morphology of the 211-particles, dislocations and unidentified nanoscale defects in the Pd-doped samples, which imply that small amounts of Pd additions can affect the microstructure as well as altering the pinning mechanism.

MATERIALS CHARACTERISTICS OF A SINGLE GRAINED Y-BA-CU-O SUPERCONDUCTOR

Single grained Y–Ba–Cu–O superconducting material has demonstrated its unique magnetic properties, such as strong flux trapping (up to 10 T at 40 K) and strong flux shielding (up to 18 T at 4 K) by eliminating the grain boundary weak link and improving the flux pinning effect. This improvement of magnetic properties in YBCO superconductors is due to the advance of materials processing techniques, i.e., top-seeding and melt-texturing method. With the TSMT method, single grained YBCO samples up to several centimeters in diameter can be produced with very high reproducibility. In this report, the TSMT method will be introduced. The results of the TSMT-YBCO sample’s superconducting properties, i.e., magnetic field intensity and distribution uniformity, levitation force, etc., as well as their correlation with various materials processing methods will be discussed.

A Superconducting Magnetization System With Hybrid Superconducting Wire for the Study and Application of Superconductivity

For the study of high-temperature superconductors and for the application of superconductivity, a superconducting high-field magnetization system with a hybrid superconducting coil was designed. This coil is composed of superconducting wires of three kinds-HTS YBCO 2G-wire, high-field and low-field NbTi wires. Three principal purposes of building this system are to inspect the characteristics of disk-shape bulk YBCO, to develop a HTS-bulk undulator, and to magnetize a portable HTS-bulk high-field magnet for application to a resonant X-ray scattering experiment. With four steps of temperature decrease, a two-stage GM-type cryocooler and liquid nitrogen provide cooling for the magnetization system. The hybrid superconducting coil, two cryogenic systems for the magnetization system and the portable HTS-bulk magnet, and the control and monitor systems that were designed are discussed.

Study of Micro/Nano Scale Displacement by Electro-Maglev System Using Bulk Superconductors and Permanent Magnet

In order to control the displacement of devices within a micro/nano scale, in this study, an active-magnetic levitation (maglev) system composed of a single-grained Y-Ba-Cu-O high temperature superconductor (HTS) bulk, a rectangular Nd-Fe-B permanent magnet (PM), and a Helmholtz coil was constructed. It is well known that the magnet can be levitated stably above superconductors by the flux trapping effect. By changing the current in the Helmholtz coil, the additional magnetic field was used to adjust the levitation height of the PM. The experimental results show that a D.C. power supply with the sensibility of 10-2 ampere was able to adjust the levitation height to be accurate within a micrometer (mum) range. An optic-fiber displacement sensor was used to measure the micro/nano scale displacement. The linear relationships between applied current, field cooling height, and non-contact displacement were determined and will be discussed in this report.

Effect of different nanoscale RE2BaCuO5 additions on the formation of compositional fluctuation in Sm-Ba-Cu-O superconducting bulk materials

This study presents the effect of different nanoscale RE211 additions—Y 2 BaCuO 5 (nmY211), Sm 2 BaCuO 5 (nmSm211), and Nd 4 Ba 2 Cu 2 O 10 (nmNd422)—on the nano-scale compositional fluctuation and associated pinning mechanism of the melt-textured growth (MTG) Sm–Ba–Cu–O [SmBCO, of which the composition is Sm123 (Sm 1 Ba 2 Cu 3 O y ) + 25 wt% Sm211 (Sm 2 BaCuO 5 )] superconducting bulk materials. The superconductivity and microstructure results indicated that in samples with the addition of these nano-sized particles, nanoscale compositional fluctuations form during the peritectic transformation of melt-growth process, which cause T c variation on a nanoscale and result in the formation of δ T c pinning centers at high magnetic field. The compositional fluctuation regions (δ T c pinning centers) are altered by the difference in peritectic temperature, the solubility in the liquid phase, and the ion radius. The direct current transport R - T properties elucidate the change of flux pinning behavior. In addition, the different influence on microstructure and superconductivity between the two methods: mixing rare-earth elements in nanoscale RE211 or in the homogeneous mixed precursor powders [e.g., (Nd, Eu, Gd)–Ba–Cu–O (NEG)] is also discussed.