Y. S. Sung

Specific heat capacity and hemispherical total emissivity of liquid Si measured in electrostatic levitation

Containerless solidification of Si was carried out via electrostatic levitation. Typical undercooling up to 370 K, which is 0.22 of its melting temperature, was obtained. A t(T) function in a time–temperature profile was analytically determined from radiative cooling curves measured during containerless solidification. With the analytical t(T) function, the ratio of constant pressure specific heat capacity to hemispherical total emissivity cP(T)/eT(T) was estimated. cP(T) and eT(T) were further derived to describe the temperature-dependent cooling behavior of liquid Si during containerless solidification in electrostatic levitation, which can be expressed as cPdrv(T)=3R+4.8×10−4⋅T+4.157×105⋅T−2−1.002×10−7⋅T2 (J/mol K) and eTdrv(T)=0.267−5.615×10−5⋅T+9.133×10−9⋅T2, respectively.

Containerless solidification of Si, Zr, Nb, and Mo by electrostatic levitation

Containerless solidification of Si (Tm=1687 K), Zr (Tm=2128 K), Nb (Tm=2750 K), and Mo (Tm=2896 K) has been carried out by the electrostatic levitation (ESL) process in which a bulk sample was electrostatically charged and levitated, then melted by laser irradiation in high vacuum. Multiple cycles of melting–undercooling–recalescence–cooling of Si (23.1 mg), Zr (41.9 mg), Nb (40.4 mg), and Mo (45.7 mg) were reproducibly repeated. Among them, Mo is a refractory material with the highest melting temperature ever containerlessly melted and solidified by electrostatic levitation. The ESL-processed Mo sample showed a thermal dendritic structure throughout the entire surface uniformly without any apparent grain boundaries implying heterogeneous nucleation must have been suppressed due to the containerless condition in the ESL process. The back reflection Laue pattern also revealed a homogeneous quality of the ESL-processed Mo sample.

Spherical Nb single crystals containerlessly grown by electrostatic levitation

Spherical Nb (Tm=2750 K) single crystals were grown via containerless electrostatic levitation (ESL). Samples became spherical at melting in levitation and undercooled typically 300–450 K prior to nucleation. As-processed samples were still spherical without any macroscopic shape change by solidification showing a uniform dendritic surface morphology. Crystallographic {111} planes exposed in equilateral triangular shapes on the surface by preferential macroetching and spotty back-reflection Laue patterns confirm the single crystal nature of the ESL-processed Nb samples. No hysteresis in magnetization between zero field and field cooling also implies a clean defect-free condition of the spherical Nb single crystals.

Deriving thermophysical properties of undercooled liquid Zr from radiative cooling curves measured by containerless electrostatic levitation

An analytical way of deriving constant pressure specific heat cP(T) and hemispherical total emissivity eT(T) of materials in the undercooled liquid state is presented. A t(T) function in a time-temperature profile was analytically determined from the radiative cooling curves of Zr measured by electrostatic levitation (ESL) in which a bulk sample was electrostatically charged and levitated then melted by laser irradiation in high vacuum. The cooling behavior of liquid Zr was found to be well described by an inverse bi-quadratic polynomial of t(T). The cP(T)/eT(T) ratio was then estimated, and eT(T) and cP(T) were calculated; they were found to be consistent with the reference data in the literature. This implies that the rationale behind the analytical approach taken in this study is reasonable and can be applied to estimating the cP(T)/eT(T) ratio, thus further facilitating derivation of eT(T) and cP(T) of various materials.

E–J characteristics of Bi-2212/Ag and Bi-2223/Ag tape conductors

Abstract We evaluated the E – J characteristics of dip-coat-processed and powder-in-tube (PIT)-processed Bi-2212/Ag and Bi-2223/Ag tapes by both the resistive and the magnetization methods. The slopes of the magnetically investigated log E –log J curves ( n -values) were almost equal to the slopes of resistively measured n -values for dip-coat-processed Bi-2212 and Bi-2223 tapes. For the PIT-processed tapes, however, the n -values obtained by the resistive method were smaller than those obtained by the magnetization method. This smaller resistively measured n -values of the PIT-processed tapes should be attributed to their inferior microstructure. No substantial difference in magnetically investigated n -values was observed between dip-coat-processed and PIT-processed tapes, which suggests that the pinning characteristics are the same for tapes prepared by these two methods. N -values of ∼20 were obtained in 10 T for dip-coat-processed Bi-2212 tapes, while larger n -values of ∼34 were obtained for dip-coat-processed Bi-2223 tapes although the J c values of the Bi-2223 tapes were much lower than those of the Bi-2212. This suggests that Bi-2223 tape is also a good candidate for the conductors of a high-field magnet operated under a persistent current mode.

Deriving thermophysical properties of liquid Nb by containerless electrostatic levitation

Containerless solidification of Nb was carried out by the electrostatic levitation process in which a bulk sample was electrostatically levitated and melted by laser irradiation in high vacuum. A t(T) function in a time-temperature profile was analytically determined to describe the radiative cooling curves of Nb in the liquid state to estimate the ratio of constant pressure specific heat to hemispherical total emissivity, cP(T)/eT(T), and further to derive eT(T) and cP(T). The cooling behavior of liquid Nb was found to be well described by an inverse biquadratic polynomial of t(T). With the analytical t(T) function, the derivative dt/dT was easily obtained and the cP(T)/eT(T) ratio was readily estimated from the radiative cooling curve. The estimated cP(T)/eT(T) and the derived eT(T) and cP(T) agreed well with the reference data in the literature. This implies the rationale behind the analytical approach taken in this study was reasonable and can be applied to estimating cP(T)/eT(T) and further deriving ...

Erratum: “Deriving thermophysical properties of liquid Nb by containerless electrostatic levitation” [J. Appl. Phys. 92, 6531 (2002)]

TABLE I. Properties of Nb. The surface area was calculated from the density ~see Ref. 8! and the final weight of a Nb sample after a series of ESL cycles. « T(T) was obtained by curve fitting the reference data ~see Ref. 14!. R is the gas constant. Tm ~K! 2750 a a !

Superconducting properties of single-crystal Nb sphere formed by large-undercooling solidification process

Abstract An electrostatic levitation (ESL) system has been used for investigating undercooling effects on superconducting materials. In this report, preliminary experiments on Nb (melting temperature: T m =2477 °C) have been performed by melting Nb in levitation using 150 and 250 W Nd-YAG lasers. Since molten Nb is solidified without any contact in a high vacuum condition, a significantly undercooled state up to 400 °C is maintained before recalescence followed by solidification. Spherical single crystals of Nb are formed by the ESL process due to the suppression of heterogeneous nucleation. The field dependence of magnetization of Nb shows a reversible behavior as an ideal type II superconductor, implying that it contains almost no flux-pinning centers.

Ideal type II superconductivity of single-crystal niobium spheres solidified from the large undercooled state

We have developed an electrostatic levitation furnace for performing a containerless solidification process on superconductors. Spherical niobium is solidified from the large undercooled state at approximately around 400°C below the melting temperature. The morphologies show homogeneous cellular structures over the entire surface. X-ray Laue photographs indicate that the spherical niobium is composed of a single grain. Magnetization below the superconducting transition temperature shows perfect diamagnetism in the low magnetic field independent of whether loading is carried out under field cooling or zero-field cooling conditions. The field dependence of magnetization shows almost reversible behavior as that of an ideal type II superconductor. These results suggest that the spherical niobium formed from the large undercooled state in this study has a very small number of flux-pinning centers compared to that formed by conventional arc melting.

CONTAINERLESS SOLIDIFICATION OF REFRACTORY METALS BY ELECTROSTATIC LEVITATION

We have been developing an electrostatic levitation (ESL) system to investigate the new process of containerless solidification with significant undercooling for researching new materials or metastable phases. In this report, preliminary experiments for three refractory metals Zr Nb and Mo have been performed by the ESL process using 150 and 250 W Nd:YAG (Naodymium:Yttrium Aluminium Garnet) lasers. Since molten metals are solidified without contact in a high-vacuum condition, a significant undercooled state up to 300–500°C is maintained before recalescence followed by solidification. Dendritic growth of these refractory materials of Nb and Mo is also driven with suppression of heterogeneous nucleation. The ESL-processed Nb showed a superconducting transition at 9.1K, which is higher than as-arc melted Nb at 8.6K. The magnetization curves imply that it contains fewer pinning centers.