Kensuke Higuchi

Precise measurement of density and structure of undercooled molten silicon by using synchrotron radiation combined with electromagnetic levitation technique

X-ray diffraction and density measurements have been simultaneously performed to investigate the atomic structure of molten silicon in a wide temperature range including the undercooling region by using the electromagnetic levitation technique. The density was obtained from the mass and the shape of a levitated sample by a non-contact method based on the image analysis technique. X-ray diffraction experiments were performed by using the synchrotron radiation at SPring8, Japan. From structural analysis of undercooled molten silicon, the first nearest neighbour coordination numbers and interatomic distances were about 5 and 2.48 A with no dependence on temperature in the range of 1900–1550 K. We conclude as a result that the short-range order based on tetrahedral bonds of undercooled molten silicon does not change with the degree of undercooling but that medium-range order changes by the degree of undercooling.

Does supercooled liquid Si have a density maximum

We have performed precise measurements of the density of supercooled liquid silicon (l-Si) in the temperature range of 1530-1800 K using an electromagnetic levitation (EML) technique with static magnetic fields. We also performed first-principles molecular dynamics simulation (FPMD) of supercooled l-Si. The observed density of the supercooled l-Si and the FPMD results show good agreement in the temperature range of 1530-1800 K. The structure of the supercooled l-Si also showed good agreement between the experimental measurements and FPMD simulations. Based on these results, we discuss nucleation in supercooled l-Si and also the existence of a density maximum in the supercooled l-Si, which is well known in water at 4 degrees C.

Compact electrostatic levitator for diffraction measurements with a two axis diffractometer and a laboratory x-ray source

A compact electrostatic levitator was developed for the structural analysis of high-temperature liquids by x-ray diffraction methods. The size of the levitator was 200 mm in diameter and 200 mm in height and can be set up on a two axis diffractometer with a laboratory x-ray source, which is very convenient in performing structural measurements of high-temperature liquids. In particular, since the laboratory x-ray source allows a great amount of user time, preliminary or challenging experiments can be performed with trial and error, which prepares and complements synchrotron x-ray experiments. The present small apparatus also provides the advantage of portability and facility of setting. To demonstrate the capability of this electrostatic levitator, the static structure factors of alumina and silicon samples in their liquid phases were successfully measured.

Real-time x-ray observation of solidification from undercooled Si melt

Grain refined microstructure is often obtained spontaneously in the solidification of metals and semiconductors from the undercooled melt without any external forces. Although it has been reported that the grain refinement is mainly caused by the fragmentation of the dendrites, the dynamic process of the fragmentation of dendrites has not been fully understood because only the microstructure after the solidification has been analyzed. Here, we present a time-resolved two-dimensional x-ray diffraction experiment on the solidification of Si from the undercooled melt. The number of diffraction spots observed at low undercoolings (ΔT<100K) did not increase at the plateau stage, while the diffraction pattern at medium undercoolings (100K<ΔT<200K) changed from the spots with the tail to rings with the lapse of time. Both this result and high speed video imaging suggested that the high-order arms of the dendrites mostly detached from the main stems because nucleation could not be expected at the melting point af...