Ceramics-Silikáty 31, (3) 227 - 236 (1987)

Byla proměřena tepelná vodivost 20 ohnivzdorných plynosilikátových vzorků SILTONTHERM na bázi kalciumsilikátů objemové hmotnosti 0,28 - 0,72 g/cm³ do teploty 900 °C. Při všech teplotách byla zjištěna lineární závislost mezi měrnou tepelnou vodivostí a objemovou hmotností. Tepelná vodivost se dá popsat známými modely pro tepelný transport v porézních materiálech a vypočítat ze strukturních dat. Též je možné stanovit pomocí modelu vliv velikosti pórů zvláště při vyšších teplotách a vypočítat měrnou tepelnou vodivost ze znalosti radiačních vlastností tepelně izolačních materiálů.

Porosity mean pore size and specific thermal conductivity at temperatures up to 900 °C by the heating wire method were measured on 20 samples with apparent densities over the range of 280 to 720 kg m⁻³. A linear dependence of apparent density on specific thermal conductivity was determined at all the temperatures. Using regression analysis, the constants of the linear dependence for the experimental values, the regression coefficients and the correlation coefficients were calculated. The effect of apparent density on specific thermal conductivity decreases with increasing temperature. At temperatures of 800 to 900 °C, the specific thermal conductivity no longer depends on apparent density. The standard deviations of the experimental values is small, increasing from 0.0057 Wm⁻¹K⁻¹ at 20 °C up to 0.0095 Wm⁻¹K⁻¹ at 900 °C. The values of specific thermal conductivity in terms of apparent density at 20 °C are 0.08 to 0.18 Wm⁻¹K⁻¹ and at 900 °C attain values from 0.27 to 0.32 Wm⁻¹K⁻¹. The thermally insulating properties of SILTONTHERM products at high temperatures are significantly affected by radiation in pores, particularly in highly porous samples. Using models for heat transfer is heterogeneous bodies, the specific thermal conductivity of the cellular concretes was calculated. It was found that the model equations derived by Loeb, Eucken Russel, Ribaud and Traustel provide calculated values that lie within the dispersion interval of the experimental values. The thermal conductivity values calculated from Roussey's model are too high, while those obtained according to Torkar and by Eucken's simplified equation are too low. The specific thermal conductivity of SILTONTHERM can therefore be calculated with satisfactory accuracy with the use of model equations and the properties of cellular concrete, most conveniently according to Loeb's model. The thermal conductivity of cellular concrete is little affected by pore size at room temperature. However, it increases considerably at high temperatures with samples containing large pores. For example, the thermal conductivity at 900 °C amounts to 0.18 Wm⁻¹K⁻¹ for cellular concrete with a mean pore size of 0.5 mm and increases up to 0.46 Wm⁻¹K⁻¹ for a mean pore size of 2 mm. This difference is also explained by radiation heat transfer in the pores. The dependence of specific thermal conductivity of SILTONTHERM on pore size can be calculated by means of Loeb's model. The cellular concrete shold have a mean pore size ofless than 1 mm to be aplicable as refractory thermal insulation.