The lattice and radiation conductivities have been determined for commercial mould fluxes in glassy and partially crystalline states as functions of the degree of crystallinity to confirm whether or not more crystallisation of mould fluxes is always effective in slow cooling in continuous casting. Lattice conductivities, refractive indices and absorption/extinction coefficients were measured on glassy and partially crystallised samples from commercial mould fluxes. The lattice conductivities of mould fluxes increased with increasing the degree of crystallinity at temperatures around 773 K and more prominent increase was observed where the degree of crystallinity exceeded about 20 %, which would be due to the contact between crystal grains precipitated. However, fluxes having higher degrees of crystallinity showed negative temperature coefficients in the lattice conductivities, particularly at higher temperatures, and thus there was a case where the lattice conductivities decreased with increasing the degree of crystallinity at higher temperatures. On the other hand, the radiation conductivities tended to decrease with increasing the degree of crystallinity and became almost constant where the degree of crystallinity exceeded about 15 %. As a consequence of this, more crystallisation does not always lead to slow cooling in continuous casting: the degree of crystallinity should be controlled to be about 15 % where the partially crystalline phase exists around 773 K.KEY WORDS: lattice conductivity; radiation conductivity; mould flux; crystallisation; hot wire method. air gap i.e., a thermal resistance, at the mould/flux interface 2) due to the fact that the density of the crystalline phase is greater than that of the glass.Thus, the degree of crystallinity of the flux film is a key factor in the control of horizontal heat flux. Consequently, when casting slabs of MC steel, it is customary to use a mould flux which gives a film with a high degree of crystallinity to reduce the horizontal heat transfer. However, when casting round billets, it is important to minimise the shrinkage of the flux film to support the billet and in this case it is necessary to use a flux giving a glassy film.3) The heat transfer is reduced by introducing transition metal oxides, which increases the absorption coefficient a and thus decreases l R . Paradoxically, l L increases as the degree of crystallinity increases because of the increased packing density and regularity in crystal structure but the enhancement of the total heat transfer across the mould flux by these effects would be much smaller than the decrease in the heat transfer due to (i) pore and air gap formations and (ii) the reduction of l R . The present work primarily focuses on qualitative investigation about the effect of crystallisation on lattice and radiation conductivities for mould fluxes. From such a viewpoint, several attempts have been made to measure values of l L of silicates in liquid and solid states, [4][5][6][7][8][9][10] suggesting that more crystallised samp...

No abstract

The thermal diffusivity/conductivity of FeO scales produced on iron substrates by thermal oxidation have been determined as functions of temperature. Iron plates (99.99%) were oxidised at 973 K in air to obtain oxide scales of FeO, Fe 3 O 4 , and Fe 2 O 3 , and then were reduced at 1 273 K in nitrogen to obtain FeO-only. The densities of these scales were 5.85-6.05 g cm − 3 . The laser flash method was used to measure the apparent thermal diffusivity of the whole sample from room temperature to 1 164 K during the heating/ cooling cycles. This was converted to the thermal diffusivity of the scale only, which in turn was converted to the thermal conductivity. However, these values depended on the scale thickness, which suggests an interfacial heat resistance occurs between the scale layer and iron substrate. In addition, scanning electron microscopy (SEM) observations revealed that the scales contained Fe and Fe 3 O 4 phases after heating. The scale thermal diffusivity/conductivity were corrected considering the interfacial heat resistance and dispersed phases to derive the corresponding values for FeO only. The interfacial heat resistance derived from the thickness dependence of the scale thermal conductivity was 8.3 × 10 − 6 m 2 K W − 1 . Using this value, the thermal diffusivity of FeO was derived as 3.7 × 10 − 7 -5.8 × 10 − 7 m 2 s − 1 and the thermal conductivity as 1.8-2.5 W m − 1 K − 1 between room temperature and 1 164 K. The temperature coefficients of the thermal conductivity were mostly negative, which would be dominated by the phonon mean free path.

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.

Apparent reflectivities and transmissivities have been measured as functions of cuspidine grain diameter for mould fluxes having constant degrees of crystallinity. Samples used were two types of synthesised mould flux with the basicity of 1, one of which samples contained 1 mass% of Fe2O3, and the grain diameter was varied in the range 1-3.5 μm. The optical measurements were carried out in the wavelength range 300-2 600 nm at room temperature using a spectrophotometer with an integrating sphere. With increasing grain diameter, the apparent reflectivity tended to increase and the apparent transmissivity tended to decrease at higher wavelengths for iron oxide free mould fluxes: it seemed that the apparent reflectivity showed a maximum value and the apparent transmissivity showed a minimum value in the grain diameter range 2-3 μm. In contrast, there was less significant dependence on grain size for mould fluxes containing iron oxides. The total radiative heat flux which may reach the mould from the steel shell has been evaluated using apparent reflectivity and transmissivity data on the basis of an optical process model. It has been found that the total radiative heat flux would be smallest in iron oxide free mould fluxes having the highest apparent reflectivity and the lowest apparent transmissivity at higher wavelengths. Effects of grain size on the radiative heat flux are smaller for mould flux containing iron oxides. Comparison of the total radiative heat flux with the total heat flux including conductive contribution suggests that control of cuspidine grain diameter would lead to reduction of the total heat flux by 7-8% for iron oxide free mould fluxes. In addition, the air gap layer would affect the total heat flux more efficiently where the volume fraction of air in the layer exceeds 85%.