Rie Endo

Electric Resistivity Measurements of Sb2Te3 and Ge2Sb2Te5 Melts Using Four-Terminal Method

In this work, we aim to determine the electric resistivities of liquid Sb2Te3 and Ge2Sb2Te5. Electric resistivities were measured using the four-terminal method. First, the electric resistivities of liquid Ga and Sn were measured to establish this method. Second, the electric resistivities of Sb2Te3 and Ge1.6Sb2.0Te5.0 were measured over temperature ranges between the respective melting temperatures of samples and 1020 K. The electric resistivity of Sb2Te3 has been determined to be 4.36±0.14 µΩ m at 992 K. The uncertainty was determined on the basis of the guide to the expression of uncertainty in measurement. The electric resistivity of Ge1.6Sb2.0Te5.0 is smaller than that of Sb2Te3. It is also found that both resistivities decrease with an increase in temperature; which indicates that both liquid materials behave as a semiconductor. Therefore, the pseudogap model was applied to derive the electrical activation energies.

Thermal Conductivity Measurements of Solid Sb2Te3 by Hot-Strip Method

The thermal conductivity of solid Sb2Te3 has been measured by the hot strip method from room temperature to 789 K. The thermal conductivity of solid Sb2Te3 decreases with an increase in temperature up to approximately 500 K and then increases. It is proposed that phonon thermal conduction dominates the heat transport around room temperature although other mechanisms such as electron and ambipolar diffusion are also in operation. Ambipolar diffusion is thought to be more dominant at higher temperature and contributes to the increase in the thermal conductivity.

Electrical and heat conduction mechanisms of GeTe alloy for phase change memory application

GeTe alloy has drawn much attention as one of the promising candidates for phase change memory application. In this work, the electrical resistivities and thermal conductivities of GeTe alloy have been determined as functions of temperature by the four-terminal method and hot strip method, respectively. The electrical resistivity increases and the thermal conductivity decreases monotonically with increasing temperature, and thus it is likely that free electron dominates the thermal conduction. The electrical resistivity increases slowly with time during holding at 773 K, and the thermal conductivity decreases corresponding to the change of the electrical resistivity, which suggests that small amount of high temperature phase might exist in the samples.

Thermal conductivities and conduction mechanisms of Sb-Te Alloys at high temperatures

Sb-Te alloys have drawn much attention due to its application in phase change memory as well as the unique properties as chalcogenide. In this work, the thermal conductivities of Sb-x mol%Te alloys (x = 14, 25, 44, 60, 70, and 90) have been measured by the hot strip method from room temperature up to temperature just below the respective melting points. For the intermetallic compound Sb2Te3 (x = 60), the thermal conductivity decreases up to approximately 600 K and then increases. For other Sb-x mol%Te alloys where x > 60, the thermal conductivities of the alloys decrease with increasing temperature. In contrast, for x < 60, the thermal conductivities of the alloys keep roughly constant up to approximately 600 K and then increase with increasing temperature. It is proposed that free electron dominates the heat transport below 600 K, and ambipolar diffusion also contributes to the increase in the thermal conductivity at higher temperatures. The prediction equation from temperature and chemical composition h...

A Reversible Change of Reflected Light Intensity between Molten and Solidified Ge-Sb-Te Alloy

The reflected light intensity (IR) from the solid and liquid phases of Ge–Sb–Te alloy has been statically measured. IR rapidly decreased after crossing the melting point during the heating process, and increased to its initial intensity level after solidifying during the cooling process. A special sample preparation technique using a quartz cell enabled reliable measurements to be carried out. The structure and composition of the Ge–Sb–Te alloy was investigated using Raman scattering and X-ray fluorescence after the melting process. Germanium was found to preferentially diffuse from the bulk to the surface leading to a germanium oxide surface phase.

Spectroscopic Ellipsometry Measurements for Liquid and Solid InSb around Its Melting Point

We have carried out spectroscopic ellipsometry measurements for liquid- and solid-phase InSb around its melting point for wavelengths of 300 to 1700 nm. The real and imaginary parts of the complex refractive index for liquid and solid InSb appear to be completely different, with the imaginary part for the liquid being higher than that of the solid. This result agrees with the proposed mechanism for the super-resolution readout effect in optical disks using an InSb film. From a dielectric function analysis, we find that InSb characteristics clearly change from semiconducting in the solid to metallic in the liquid.

Optical Measurement for Solid- and Liquid-Phase Sb2Te3 around Its Melting Point

We have developed a system for measuring the complex refractive index of liquid- and solid-phase chalcogenide around their melting points. The system consists of a spectroscopic ellipsometer, an infrared heating system, and prism optics. As a container for the chalcogenide, we use a customized quartz cell, evacuated to several pascal level to avoid sample degradation. We adopted a measurement configuration that uses access from the bottom side, because a mirror-like surface which is necessary for optical measurement was naturally and easily created at the container bottom by gravity. We succeeded in observing the remarkable difference on the indices between liquid- and solid-phase Sb2Te3.

Emissivities of High Temperature Metallic Melts

The mathematical modeling of heat flow in high temperature processes has been a useful means of obtaining more efficient process design and stricter process control. At high temperatures heat can be transferred by three mechanisms, that is, conduction, convection, and radiation. Analysis of heat transfer requires physical property data of a medium through which heat is transferred, relevant to the respective mechanisms: the thermal conductivity is indispensable to heat flux calculation by conduction from Fourier’s law, and the viscosity, density, and heat capacity are indispensable to heat flux calculation by convection. On the other hand, the emissivity plays a key role in heat transfer analysis by radiation because it quantifies how well a substance radiates energy in the form of light. The emissivity is more important in the analysis for materials processing involving metallic melts since the radiation contribution becomes more predominant as temperature rises. Thus, emissivity measurements have been conventionally attempted on various metallic melts at high temperatures, data of which have been published in the handbook [1], for example. However, the data have not been abundant enough for practical use in mathematical modeling, and measurements are still now being made continuously. This chapter focuses on emissivities of metallic melts and reviews recent measurement techniques and data for the emissivity, mainly on and after the publication of the handbook.

Reduction of Iron Oxides in Mould Fluxes With Additions of CaSi2

Iron oxides in mould fluxes enhance heat extraction from the molten steel to the mould due to energy absorption by d-d transitions of Fe2+ and re-emission. Thus, the existence of iron oxides is against mild cooling of molten steel. In this study, mould flux powders containing ca 2 mass% Fe2O3 were mixed with sufficient amounts of CaSi2. The mixtures were contained in alumina crucibles and melted at 1673 K in Ar-H2 atmosphere. The melts were poured into brass moulds to obtain glassy samples 5 mm thick. The Fe2O3 concentration was analysed by a scanning electron microscope with an energy dispersive spectrometer. The concentration decreased from ca 1.71 mass% to 0.49 mass% within 5 min and then settled down. Mass transfer of Fe2O3 is supposed to be the rate-controlling step at high temperature. The mass transfer coefficient has been calculated to be 1.8×10-3cms-1, which seems reasonable. Since crystallisation of mould flux enhance heat reflection from the molten steel to reduce heat transfer, crystallisation kinetics of mould fluxes has been investigated using the Avrami equation, which suggests that additions of reducing agents such as CaSi2 suppress the crystallisation process. In addition, CaSi2 additions result in a dramatic decrease in the total radiative heat flux across mould fluxes in glassy state and would be effective for mild cooling of molten steel.

Effects of CaF 2 on the Radiative Heat Transfer in Mould Fluxes for Continuous Steel Casting

Effects of CaF2 additions to mould fluxes have been investigated from the perspective of radiative heat transfer reduction to design mould flux for mild cooling. Glassy and crystallised mould flux samples were prepared so that the basicity of CaO/SiO2 was 1 and the Fe2O3 concentration was 1mass%, whereas the concentration of fluorine ranged between 2 and 14mass%. The samples were analysed by SEM-EDS and XRD. The apparent reflectivities and transmissivities were measured using two types of spectrophotometer with an integrating sphere. The replacement of CaO by CaF2 gives no effect on the optical characteristics of the glassy samples, not leading to radiative heat transfer reduction. In contrast, the replacement affects the optical characteristics of the crystallised samples and also the radiative heat transfer, which appear due to changes in the crystalline phases produced and the degree of crystallinity rather than the interaction between iron and fluoride irons.