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

Abstract

Using a transient hot-wire method, the thermal conductivity of the CaO-SiO2-B2O3 mold flux system was measured. The effects of temperature, BO1.5 concentration and basicity on the thermal conductivity were considered, along with structural investigation by Raman spectroscopy. It was found that the addition of boron oxide caused both a decrement and increment of thermal conductivity, depending on the basicity. These conflicting effects on thermal conductivity were considered to be caused by the following two different behaviors in the oxide melts. Boron oxide is incorporated into silicate networks at a lower basicity, while it tends to form borate networks at higher CaO/SiO2 ratios. In the case of basicity dependency, thermal conductivity initially decreases or remains constant with increasing CaO/SiO2 ratio in regions of low basicity, but increases when the CaO/SiO2 ratio is higher than 1.15. Due to the incorporated state of boron oxide in silicate networks at low basicity, the thermal conductivity is likely to be predominantly affected by the silicate networks. However, at a relatively high CaO/SiO2 ratio, an increase in chain-type metaborate was observed through Raman spectroscopy; this structural change in borate being responsible for the increment in thermal conductivity with higher basicity. Finally, the apparent activation energy of thermal conductivity was calculated, and was found to be reduced by the addition of boron oxide.

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

“…Relative uncertainty Temperature calibration 0.2% Accuracy of standard resistor 0.01% Resolution of digital multimeter 0.002% Heat by radiation 0.8% The detailed experimental conditions and procedures of the hotwire method have been described elsewhere [26,27].…”

“…Although Kim and Morita [26] measured thermal conductivity in the CaO-SiO 2 -B 2 O 3 mold flux system, it mainly consisted of the metaborate units formed by asymmetric 3-coordinated boron due to the high CaO concentration and the low B 2 O 3 concentration. For this reason, the effect of 4-coordinated boron on thermal conductivity could not be appropriately evaluated along with silicate network structure.…”

“…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.)…”

Influence of Al2O3, CaO/SiO2, and B2O3 on Viscous Behavior of High Alumina and Medium Titania Blast Furnace Slag

“…So the slag structure becomes simpler and less complex. This is attributed to a weakening of the bond energy by increasing of B-O bonds in the network structure of BO 4 and BO 3 units [30]. In other words, although increasing Al 2 O 3 can make the network of slag melts complex, it was modified by adding B 2 O 3 .…”

Structure and Properties of Slags Used in the Continuous Casting of Steel: Part 1 Conventional Mould Powders

“…There is a huge discrepancy between thermal conductivity values derived using the THW and LP methods with k THW = 0.1 to 0.3 57,80,[92][93][94] and k LP ≈1.5 Wm ). The k values for the liquid have been reported to increase (i) with increasing polymerisation of the silicate network (i.e.…”