Hidekazu Kobatake

Noncontact measurement of thermal conductivity of liquid silicon in a static magnetic field

Thermal conductivity of liquid silicon is indispensable for numerical modeling of silicon crystal growth processes and for elucidating electron transport phenomena in high-temperature liquids. However, crucial obstacles render measurement of thermal conductivity difficult: convection and contamination from contact materials. In this study, the authors developed a noncontact measurement of thermal conductivity of liquid silicon using electromagnetic levitation in a static magnetic field. Convection in the levitated silicon becomes negligible above 2T. The determined thermal conductivity shows that the electron contribution is dominant for thermal transport in liquid silicon at temperatures of 1750–2050K.Thermal conductivity of liquid silicon is indispensable for numerical modeling of silicon crystal growth processes and for elucidating electron transport phenomena in high-temperature liquids. However, crucial obstacles render measurement of thermal conductivity difficult: convection and contamination from contact materials. In this study, the authors developed a noncontact measurement of thermal conductivity of liquid silicon using electromagnetic levitation in a static magnetic field. Convection in the levitated silicon becomes negligible above 2T. The determined thermal conductivity shows that the electron contribution is dominant for thermal transport in liquid silicon at temperatures of 1750–2050K.

Noncontact modulated laser calorimetry in a dc magnetic field for stable and supercooled liquid silicon

Thermal conductivity of liquid silicon is necessary for numerical process modeling. It is also of scientific interest. However, measuring thermal conductivity is a difficult task because of convections in the liquid and contamination from contact materials. To overcome these experimental difficulties, we have developed noncontact modulated laser calorimetry in a dc magnetic field to measure heat capacity, thermal conductivity and emissivity of high-temperature liquid metals. In this study, through improvement in temperature measurements, we considerably reduced the experimental uncertainty of measurements. Furthermore, the thermal conductivity and heat capacity of supercooled liquid silicon were measured. Thermal conductivity of liquid silicon agrees with the values calculated assuming the Wiedemann–Franz law near the melting point. This result suggests that free electrons play a dominant role in the thermal transport process in liquid silicon.

Nitridation behavior of sapphire using a carbon-saturated N2–CO gas mixture

The authors previously developed a sapphire nitridation method using carbon-saturated N2–CO gas mixture to form a high-quality AlN film for III-nitride-based optoelectronic devices. In this study, the nitridation behavior of (0001) (c) plane and (112¯0) (a) plane sapphire was studied to elucidate and optimize the process at temperatures of 1823 and 1873 K. The AlN film thickness, surface morphology, crystal quality, and interfacial phenomena were investigated as functions of nitridation time and temperature. Fundamentally, the AlN film grows as a result of the diffusion process that occurs in the AlN film. The voids found at the AlN/sapphire interface indicate that the Al2O3 dissociates into Al3+ and O2− ions, and that the ions diffuse in the AlN film. However, the growth rate of AlN film does not obey the simple diffusion model. The AlN film thickness has a maximum and decreases slightly with time, which indicates that the thermal decomposition of AlN film must be considered when comprehensively describing the nitridation process.The authors previously developed a sapphire nitridation method using carbon-saturated N2–CO gas mixture to form a high-quality AlN film for III-nitride-based optoelectronic devices. In this study, the nitridation behavior of (0001) (c) plane and (112¯0) (a) plane sapphire was studied to elucidate and optimize the process at temperatures of 1823 and 1873 K. The AlN film thickness, surface morphology, crystal quality, and interfacial phenomena were investigated as functions of nitridation time and temperature. Fundamentally, the AlN film grows as a result of the diffusion process that occurs in the AlN film. The voids found at the AlN/sapphire interface indicate that the Al2O3 dissociates into Al3+ and O2− ions, and that the ions diffuse in the AlN film. However, the growth rate of AlN film does not obey the simple diffusion model. The AlN film thickness has a maximum and decreases slightly with time, which indicates that the thermal decomposition of AlN film must be considered when comprehensively describi...

Noncontact modulated laser calorimetry of liquid silicon in a static magnetic field

Accurate thermal transport properties of high-temperature liquid silicon, such as those of heat capacity, emissivity, and thermal conductivity, are required for improving numerical modeling to produce high-quality silicon crystals using the Czochralski method. However, contamination from contact material and convection complicates measurements of these properties. The authors developed a noncontact modulated laser calorimetry using electromagnetic levitation in a static magnetic field. The isobaric molar heat capacity, thermal conductivity, and hemispherical total emissivity of liquid silicon were measured simultaneously at temperatures of 1750–2050 K. Convection in the levitated liquid silicon was suppressed above a static magnetic field of 2 T.Accurate thermal transport properties of high-temperature liquid silicon, such as those of heat capacity, emissivity, and thermal conductivity, are required for improving numerical modeling to produce high-quality silicon crystals using the Czochralski method. However, contamination from contact material and convection complicates measurements of these properties. The authors developed a noncontact modulated laser calorimetry using electromagnetic levitation in a static magnetic field. The isobaric molar heat capacity, thermal conductivity, and hemispherical total emissivity of liquid silicon were measured simultaneously at temperatures of 1750–2050 K. Convection in the levitated liquid silicon was suppressed above a static magnetic field of 2 T.

Development of modulated laser calorimetry using a solid platinum sphere as a reference

This study is a fundamental investigation aimed at developing a new noncontact modulated laser calorimetry method incorporating a static magnetic field to achieve measurement of the true thermal conductivity of a metallic melt. For establishing the experimental principle, a solid platinum sphere was used in this study. The sphere positioned in the centre of a radio frequency coil was heated sinusoidally by a laser; its temperature response was monitored. Analyses of the temperature amplitude and phase difference between the laser input and temperature response yielded the heat capacity, thermal conductivity and hemispherical total emissivity of platinum for temperatures of 1400–1700 K. The obtained data agree with the reference data within experimental uncertainty, which verifies the experimental method.

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.

Thermal conductivity measurement of molten copper using an electromagnetic levitator superimposed with a static magnetic field

The thermal conductivity of molten copper was measured by the periodic laser-heating method, in which a static magnetic field was superimposed to suppress convection in an electromagnetically levitated droplet, to extend the measurement range of the method up to a relatively high thermal conductivity. Before measuring the thermal conductivity, the optimum conditions for static magnetic field, the laser frequency of periodic heating and sample diameter were investigated by numerical simulation both for the flow and thermal fields in an electromagnetically levitated droplet and for the periodic laser heating of the droplet in the presence of melt convection. As a result, the temperature dependence of the thermal conductivity of molten copper was proposed in the temperature range between 1383 and 1665 K. In addition, by comparing our results with those of previous studies, it was demonstrated that the present method of measuring thermal conductivity is also available for molten materials with a relatively high thermal conductivity, such as molten copper.

Noncontact Laser Modulation Calorimetry for High-Purity Liquid Iron

The heat capacity and thermal conductivity of liquid iron were measured the using recently developed method of noncontact laser modulation calorimetry. An iron sample was levitated using an electromagnetic levitator. Then the convection in the levitated droplet was suppressed to measure the thermal conductivity by the application of a dc magnetic field. High-purity iron (99.9972 mass %) prepared using an ion exchange method was used for measurements. The molar heat capacity of liquid iron at constant pressure was measured to be 45.4 ±3.2 Jmol-1K-1 (1848–1992 K) in low dc magnetic fields because a semi-adiabatic condition was achieved, assisted by the remaining convection in the liquid. The apparent thermal conductivity of liquid iron decreased concomitantly with the increasing dc magnetic field. It finally converged to 39.1 ±2.5 Wm-1K-1 (1794–2050 K) at 9 T or higher. The experimental uncertainties in the molar heat capacity and thermal conductivity are double the standard deviation.

Normal spectral emissivity of stable and undercooled liquid silicon using electromagnetic levitation in a dc magnetic field

Normal spectral emissivity of a stable and undercooled liquid (1660–1790 K) was determined for wavelengths of 780–920 nm using direct measurement of radiance of statically electromagnetically levitated liquid silicon: a dc magnetic field suppressed surface oscillation and the transitional motion of the silicon droplet. A spectrometer used for spectral radiance measurement was calibrated using a quasi-blackbody with a copper metal fixed point. The emissivity shows weak negative wavelength dependence, and negligible temperature dependence. The wavelength dependence of the normal spectral emissivity is expressible using the Drude free electron model.

Effects of static magnetic field and gas atmosphere on solidification of silicon by electromagnetic levitation

The effects of static magnetic field and hydrogen gas atmosphere on the solidification of silicon with hydrogen atom were examined using electromagnetic levitation method in hydrogen atmosphere. Since the convection in the levitated silicon melt which is caused by the electromagnetic field is restrained by a high static magnetic field, it is possible to examine a solidification of silicon from an equilibrium silicon melt with hydrogen without an inflluence of convection. The cooling rate of the melt increases with increasing hydrogen partial pressure. Since the undercooling of the melt does not change with the static magnetic field, the grain size on the surface does not change with the static magnetic field. However, the morphology of the grain surface drastically changed from flat and smooth to undulation.