Effect of Na₂O additions on thermal conductivities of CaO–SiO₂slags

Abstract

Thermal conductivities of Na 2 O-CaO-SiO 2 slags have been determined using the non-stationary hot wire method over a wide temperature range between room temperature and 1773 K, to propose a prediction equation for the thermal conductivities of this slag system. Samples used were xNa 2 O-yCaO-zSiO 2 (xzyzz5100 mol-%), where 5,x,40 mol-% and 10,y,40 mol-%. With increasing temperature up to glass transition temperatures, the thermal conductivities of the samples increased and then decreased even beyond liquidus temperatures. The thermal conductivities of slags having the same SiO 2 concentration, i.e. the same NBO/T (number of bridging oxygens per tetrahedrally coordinated atom), decreased by replacement of CaO with Na 2 O but took almost the same value at their liquidus temperatures T L , irrespective of the (%Na 2 O)/(%CaO) ratio. Furthermore, plotting the thermal conductivities against T/T L (temperature normalised by T L ) shows that the thermal conductivities in the temperature range T/T L .0 . 9 are not dependent upon the (%Na 2 O)/(%CaO) ratio as long as the SiO 2 concentration is kept constant. This finding indicates that in this system the relationship between thermal conductivities at T L and NBO/T also applies at temperatures other than T L using the temperature scale of T/T L . On the basis of this relationship, the following prediction equation has been proposed for thermal conductivities l of this slag system in liquid and supercooled liquid states: l5a[1/(T/T L )]zb, where a and b are constants specified by the concentration of SiO 2 . This equation applies, for example, in the temperature range 0 . 9,T/T L ,1 . 3 for slags containing 70 mol-%SiO 2 and 0 . 9,T/ T L ,1 . 1 for slags containing 40 mol-%SiO 2 .

Relationship between Molten Oxide Structure and Thermal Conductivity in the CaO–SiO2–B2O3 System

“…3,4) In this sense, studies pertaining to the relationship between the structure and physical properties of molten oxides have been carried out by a number of researchers with the intent to better design and optimize mold flux systems. [5][6][7][8][9][10][11][12][13] Because of the poor application of the additive rule, 14) and anomalous behavior in alkali/alkali earth oxide bearing systems, 15,16) the effect of boron oxide in mold flux systems has been given particular attention. [11][12][13][17][18][19] Wang et al 19) reported that the addition of B 2 O 3 leads to an increase number of [BO 4 ] tetrahedral units, which play an important role as network forming oxides in CaO-SiO 2 -B 2 O 3 -TiO 2 slag systems.…”

Experimental investigation into radiative heat transfer characteristics for mould fluxes containing transition oxides

“…In this study, wavelength range of 1-6 mm is emphasised on account of the characteristic of continuous casting process, actually, absorption spectrum of transition elements always expand to near ultraviolet band and infrared band, the wavelength range is 200-25000 nm. 23 S. Ozawa and M. Susa 24 and Susa et al 25 investigated the thermal conductivities of Na 2 O-CaO-SiO 2 and CaO-SiO 2 -Al 2 O 3 slag and found it increases with increasing temperature up to glass transition temperatures and then decrease even beyond liquidus temperatures. Cho et al studied the effect of the temperature on the radiative heat transfer properties of the mould fluxes and showed that there is little change in the structure of all the mould fluxes and not much difference in transmittance.…”

The effect of borate and silicate structure on thermal conductivity in the molten Na2O–B2O3–SiO2 system

“…However, above the Debye temperature, owing to the approximately constant specific heat capacity (C) and mean particle velocity (v), the thermal conductivity of the oxide system decreases with increasing temperature as a result of the decrease in the phonon mean free path of collision (l), which is proportional to the inverse of the temperature (1/T). [25][26][27][28][29]31] Hence, it can be inferred that thermal conductivity reaches the maximum value around the Debye temperature, and then it decreases with higher temperatures. Therefore, identifying the Debye temperature in the oxide system is essential to evaluate the temperature at which thermal conductivity shows the maximum and to determine which variables mainly affect the thermal conductivity at the given temperature.…”

“…Although understanding the thermal conductivity of the molten oxide is also important for the thermal processing of the glass [22], the thermal conductivity measurement in the molten oxide system has been limited because of the large convection and the radiation effects [23]. However, owing to the several modifications of the non-steady state method, the thermal conductivity has been successfully measured in the molten oxide system over the last few decades [24][25][26][27][28][29][30][31][32][33]. Similar to other physical properties of the molten oxide system, the thermal conductivity is closely related to the network structure (i.e., silicate, aluminate, and borate structure.)…”