Thermal conductivity, λ, is defined by Carniglia and Barna (1992) as:
λ (T) = ρ (T).cp(T).a(T) (Eq. 2.1)
where ρ is the bulk density, cp the specific heat, a the thermal diffusivity and T the
temperature. The unit for thermal conductivity λ is W.m-1.K-1.
Unlike the heat capacity, the thermal conductivity of heterogeneous mixtures is intensely sensitive to variations in microstructure. The governing micro structural features being intimately dependant on processing and thus largely uncoupled from composition, there is no reliable “rule of mixtures” for thermal conductivity.
Figure 2.1: Thermal conductivity of insulating fire brick and insulating castables (Carniglia & Barna, 1992). STL indicating the Service Temperature limit (in °F) of the Insulating Fire Brick (IFB)
The variation of the thermal conductivity with average body temperature for insulating fire bricks and insulating castables is consolidated in Figure 2.1.
Internal heat transportation, and with it heat insulation, in high-temperature insulating materials, are decisively influenced by the structural composition and the temperature. The effectiveness of the influenced temperature is also controlled by the structure. Consequently, the structural composition plays a dominating part. As emphasised before, high-temperature insulating materials represent heterogeneous, porous multiple phase bodies. These materials facilitate extensive internal heat transportation by means of thermal conduction and heat radiation, which can be summed up as an effective thermal conductivity: • The porosity, or bulk density, has to be adapted to the temperature of the application, or the temperature gradient, intended to be applied. The porosity required for a minimum effective thermal conductivity decreases with increasing temperature of application (Schulle & Schlegel, 1991). • Porosity exerts the main influence on the effective thermal conductivity. • In cases of pure heat conduction the gas-filled pores have a small role to
play, the solid matter structures a decisive one.
• The effective thermal conductivity depends on the thermal conductivity of the pore-free, solid phase.
• The type of pore gas and the gas pressure influence the thermal conductivity.
• The pores should be as small as possible and efforts should be made to provide micro-porosity.
• The microstructure of the solid matter should consist of loosely packed crystal structures and complicated crystal lattices with little symmetry, high defect density, as well as a substantial poly- or micro crystallinity.
• The microstructure of the solid matter should show little transmission and a high degree of absorption in the infrared wave range.
• The overall structure should not allow gas permeability or at least at only on a small scale.
2.2.5 SHRINKAGE
The shrinkage behaviour of an insulating material is used for evaluating its maximum possible temperature of application. For this reason non-reversible length modification is measured over a long period of time at constant temperatures, the material being heated up on one or all sides in an oxidising atmosphere without corrosive influences. The classification temperature or the limit of application temperature corresponds to the temperature which allows a maximum admissible amount of linear shrinkage. Most countries have established different shrinkage standards. For refractory lightweight bricks and concretes there are shrinkages of 1 to 2 % and for refractory fibres 2 to 5 %, sometimes even up to 7 %. The isothermal heating time, required for thermal treatment, also fluctuates between 4 and 24 hours (Schulle & Schlegel. 1991).
A typical refractory is based on a mixture of low shrinkage clays with a small addition of plastic clays, for example ball clay, to ease shaping during manufacture and impart high green strength before firing (Hancock, 1988).
2.2.6 STRENGTH
Kruger (1996) reported the development of castable refractories from coal fly-ash and cenospheres which have physical and chemical properties that are inherently beneficial for the manufacture of insulating refractories. Their use imparts excellent flow properties to the product, thus enhancing the placeability of monolithic linings. This phenomenon has been ascribed to the lubricating (ball- bearing) effect of the spherical particles. Insulating refractories based on coal fly- ash exhibit remarkable strength to density ratios, excellent thermal shock resistance and an improved ratio of thermal conductivity to bulk density. Most
importantly, they are far more cost-effective than competitive products. In general, the higher the proportion of cenospheres in the product, the better will be the insulation efficiency and the lower the density. Compressive strength is, however, slightly lower at higher cenospheres content. The maximum service temperature of approximately 1250 to 1300 ºC does restrict the use of cenospheres and coal fly-ash to heat insulating or lower-temperature refractories. Careful selection of the particle size distribution of the coal fly-ash or cenospheres ensures optimum particle packing and enables the manufacture of low-shrinkage refractories (Kruger, 1996).
The need for energy conservation necessitates insulating refractories with improved performance. The incorporation of cenospheres as part of the formulation has enabled the manufacture of products (Cenref) that have lower thermal conductivity and greater strength, which are lighter than the conventional Moler bricks widely used in industry. A cenosphere refractory can out-perform competitive products. Besides its superior insulation, its low apparent porosity is the most significant advantage. This is ascribed to the fact that the cenosphere refractory consists of isolated spheres lightly fused together; whereas other types of insulating refractories have interconnecting micro channels. Heat diffusion is more efficient along these micro channels than across the isolated air within the spheres. The inability of liquids to penetrate the monolithic cenosphere matrix also gives these refractories superior acid resistant properties. Service temperatures of 1300 ºC have been achieved and formulations have been developed that, at elevated temperatures, provide superior insulation to ceramic fibre. Due to their excellent in-service performance, domestically developed coal fly-ash and cenosphere refractories are gaining popularity (Kruger, 1996).