Weili Ren

Magnetic-field-assisted solvothermal growth of single-crystalline bismuth nanowires.

The magnetic-field-assisted approach has been used in the shape-controlled synthesis of single Bi nanocrystals. By tuning the magnetic field strength in the solvothermal process, Bi nanowires with dimension of 40-200xa0nm and lengths up to tens of micrometers were synthesized. Various techniques such as x-ray diffraction, scanning electron microscopy, transmission electron microscopy and Fourier transform infrared spectrometry have been used to characterize the products obtained. The results show that the magnetic field plays a key role in the crystal growth of the Bi nanowires. All nanowires were highly oriented single crystals with the growth direction along the c-axis.

Effects of a high-gradient magnetic field on the migratory behavior of primary crystal silicon in hypereutectic Al?Si alloy

Abstract The migration of primary Si grains during the solidification of Al–18 wt%Si alloy under a high-gradient magnetic field has been investigated experimentally. It was found that under a gradient magnetic field, the primary Si grains migrated toward one end of the specimen, forming a Si-rich layer, and the thickness of the Si-rich layer increased with increasing magnetic flux density. No movement of Si grains was apparent under a magnetic field below 2.3 T. For magnetic fields above 6.6 T, however, the thickness of the Si-rich layer was almost constant. It was shown that the static field also played a role in impeding the movement of the grains. The primary Si grains were refined in the Si layer, even though the primary silicon grains were very dense. The effect of the magnetic flux density on the migratory behavior is discussed.

ON NUCLEATION TEMPERATURE OF PURE ALUMINUM IN MAGNETIC FIELDS

Solidiflcation of pure aluminum without and with a magnetic fleld has been investigated by difierential thermal analysis (DTA). DTA curves showed that the nucleation temperature of pure aluminum was decreased as a magnetic fleld strength increased although its melting process was almost not in∞uenced. The nucleation suppression could be attributed to the increase of the solid- liquid interfacial energy which might originate from more orderly arrangement of atoms on the solid-liquid interface upon applying a magnetic fleld.

Design and application of differential thermal analysis apparatus in high magnetic fields.

The differential thermal analysis (DTA) apparatus has been developed for a commercial superconducting magnet. The DTA apparatus could detect the kinetics and thermodynamics of phase transformation with and without a magnetic field. Preliminary results for Al-Al2Cu eutectics are presented. The DTA curves indicate the similarity at several rates regardless of a magnetic field; however, at the same rate, melting transformation seems not to be influenced by a magnetic field, while solidification could be delayed via suppressing nucleation and crystal growth in a magnetic field. It will be believed that the DTA apparatus can be used to investigate the phase transformation of substances of interest in a magnetic field.

Formation of stray grain in cross section area for Ni-based superalloy during directional solidification

Stray grains which may appear during production of single-crystal blades, can severely deprave the performance of turbine blades. This investigation primarily explores the conditions of stray grains formed in DZ417G superalloy by using samples with varying cross sections. The effect of withdrawal velocities on the microstructure of DZ417G superalloy in directional solidification is investigated. It is observed that the growth direction of dendrites did not change, and no stray grain appears in the re-entrant corner at low withdrawal velocity. Nevertheless, the stray grains grow up when the withdrawal velocity is higher than 150μm/s. The temperature profiles in the region of the cross section change at different velocities are examined. The increase of undercooling in the corner of the cross section change leads to the formation of stray grains.