Takeshi Azami

Toxicity of Single-Walled Carbon Nanohorns

We extensively investigated in vitro and in vivo the toxicities of as-grown single-walled carbon nanohorns (SWNHs), a tubular nanocarbon containing no metal impurity. The SWNHs were found to be a nonirritant and a nondermal sensitizer through skin primary and conjunctival irritation tests and skin sensitization test. Negative mutagenic and clastogenic potentials suggest that SWNHs are not carcinogenic. The acute peroral toxicity of SWNHs was found to be quite low--the lethal dosage for rats was more than 2000 mg/kg of body weight. Intratracheal instillation tests revealed that SWNHs rarely damaged rat lung tissue for a 90-day test period, although black pigmentation due to accumulated nanohorns was observed. While further toxicological assessments, including chronic (repeated dose), reproductive, and developmental toxicity studies, are still needed, yet the present results strongly suggest that as-grown SWNHs have low acute toxicities.

The role of surface-tension-driven flow in the formation of a surface pattern on a Czochralski silicon melt

Spoke patterns on shallow silicon melts and polygonal cellular patterns on the surface of Czochralski (CZ) silicon melts were observed by CCD camera. The dark stripes of the spoke patterns in the CCD images indicate the lower temperatures. We found that the number of spokes depends on the depth of the silicon melt. When the thermocapillary flow in a 3-mm-deep silicon melt was dominant, a spoke pattern was clearly observed on the surface. On a CZ silicon surface of a 200-mm-deep melt not under a vertical magnetic field, polygonal-shape patterns were observed to move toward the melt center. However, under a vertical magnetic field, the polygonal patterns only changed to be more longitudinal but did not disappear. These observations indicate that the surface flow of a CZ melt and its instability cannot be fully suppressed by a magnetic field and that the surface flow plays a significant role in forming the surface patterns.

Thermal waves of a nonaxisymmetric flow in a Czochralski-type silicon melt

Abstract Thermal waves due to a nonaxisymmetric flow were observed at a Czochralski-type silicon-melt surface with a carbon-dummy crystal. The wave number and pattern transition of the thermal waves were investigated at various crucible rotation rates. The thermal wave number increased as the crucible rotation rate increased and the rotation rate of the thermal wave was lower than the crucible rotation rate. The nonaxisymmetric flow region for the 5″ CZ silicon melt was located at the thermal Rossby numbers of 1–10 2 and was higher than the region obtained for rotating annulus experiments.

Optical observation of solid-melt interface fluctuation due to Marangoni flow in a silicon liquid bridge

We observed fluctuation of the three-phase (solid, melt and ambient atmosphere) boundary position in silicon liquid bridges. We proved by measuring interface fluctuation and temperature fluctuation simultaneously that this interface fluctuation is caused by the formation and fluctuation of asymmetrical temperature field due to Marangoni flow instability. The amplitude of interface fluctuation decreased with increasing oxygen partial pressure. This decrease in interface fluctuation suggests that oxygen partial pressure is a factor that influences Marangoni flow in molten silicon.

Interpreting the oxygen partial pressure around a molten silicon drop in terms of its surface tension

Abstract The oxygen partial pressure around a molten silicon drop was determined from the inlet ( P O 2 inlet ) and outlet ( P O 2 exit ) pressures of a furnace under precisely controlled gas flow rate. It was found that P O 2 exit is much lower than P O 2 inlet and that measured P O 2 exit depends on gas flow rate, but P O 2 inlet does not. This dependence can be explained by an oxygen transfer model using the Peclet number.

Effect of Oxygen on Thermocapillary Convection in a Molten Silicon Column under Microgravity

X-ray flow visualization in a half-zone molten silicon bridge was carried out under microgravity at a variety of oxygen partial pressures in an ambient atmosphere. The flow velocity of thermocapillary convection was found to decrease with increased oxygen partial pressure. Thermocapillary flow stabilized when the oxygen partial pressure at the inlet PO 2 was increased to 10 4 MPa. The dependence of the flow velocity and flow mode on PO 2 can be explained in terms of the decrease in the temperature coefficient of surface tension due to the increase in oxygen partial pressure.

Marangoni flow of molten silicon

Abstract Marangoni flow of molten silicon was studied for a half-zone liquid-bridge configuration. Through flow visualization using an X-ray radiography technique with tracer particles and temperature oscillation measurements, the instability mode for the Marangoni flow was determined. It was found that m=1 and 2 modes appeared depending on the aspect ratio ( Γ= height h/ radius r ) of the liquid bridge. The critical Marangoni number for transition from an oscillatory flow with single frequency to that with multiple frequencies was deduced to be about Ma=1300, based on the calibrated-temperature difference between hot and cold solid/liquid interfaces. A transition was also observed when the oxygen partial pressure of the ambient atmosphere was changed. The flow velocity observed using a tracer particle also showed a dependence on the oxygen partial pressure; the velocity decreased with increasing oxygen partial pressure. By observing surface oscillation using a spatial-phase measurement technique, Marangoni oscillation at the melt surface was successfully distinguished from natural oscillation with eigenfrequencies. Marangoni oscillation ( >1 Hz ) that was not revealed by flow visualization and temperature measurement using thermocouples was also observed. Marangoni flow at a flat surface should be studied, so that the heat and mass transfer process for the Czochralski system can be more clearly understood and controlled.

Surface Tension Driven Flow of Molten Silicon: Its Instability and the Effect of Oxygen

Surface-tension-driven flow of molten silicon, which is one of mechanisms of heat and mass transfer during crystal growth, was investigated by using a liquid-bridge configuration under microgravity and on earth. Using microgravity is a convenient way to study surface-tension-driven flow, because buoyancy flow can be suppressed so that only surface-tension-driven flow can be distinguished. In the liquid-bridge configuration, which corresponds to floating-zone growth, flow instability and its three-dimensional structure were investigated through measurement of temperature-oscillation, flow visualization, optical pyrometry of the melt surface, observation of oscillation of the melt/crystal interface, and observation of surface oscillation by phase-shift interferometry. Azimuthal wave number m for instability structure depends on the aspect ratio of the bridge, Γ, which is defined as the ratio of height h to radius r.

Non-invasive techniques for observing the surface behavior of molten silicon

The behavior of molten silicon surfaces was observed by using non-invasive techniques such as laser microscopy, phase-shift Michelson interferometry and CCD thermometry. The formation and oscillation of a non-axisymmetric temperature field due to surface-tension-driven flow instability was revealed in a molten silicon bridge, which simulates floating zone configuration. For a flat surface, which represents a Czochralski melt, a hydrothermal wave was plausibly observed when the melt was shallow, whereas a cell pattern was observed during crystal growth when the melt was deep. The cell pattern was modified by application of a magnetic field.

The Effect of Oxygen Partial Pressure on Marangoni-Flow-Induced Dopant Striations in Floating-Zone Silicon Crystals

A silicon crystal was grown after the floating-zone (FZ) method under oxygen partial pressures (Po2 in) of 2.0×10-8 MPa and 5.8×10-6 MPa, which were measured at the entrance of a furnace. It was found that dopant striations showed a single-peak spectrum at 0.19 Hz in this crystal, and a crystal grown at Po2 in of 2.0×10-8 MPa showed multiple frequencies. This result is explained by a decrease in the Marangoni number with increasing oxygen partial pressure. It has thus been concluded that Marangoni flow in FZ silicon crystal growth can be controlled by introducing oxygen gas, independently of the temperature field control in the vicinity of the crystal/melt interface.