Mullite thermal conductivity

A schematic of an Anter radial thermal conductivity measuring instrument using this principle is shown in Figure 9.2. A specimen in the form of an annular cylinder is placed to surround a central heater. Often the cylinder is made up of a series of stacked rings. Alternatively, a granulated or fibrous form of the specimen may be poured or placed between the central heater ( 1.2 cm OD) and a mullite outer casing ( 10.5 cm ID). 

Mullite has the advantage that its lower thermal conductivity diminishes axial heat flow along the central heater more effectively. 

Kiln furniture must be made of materials resistant to considerable temperature cycling in particular, during fast firing the changes in temperature are quite rapid (heating rate up to 30 — 40 ""C min ). A low thermal expansion coefficient and/or high thermal conductivity are therefore required. These requirements are best met by cordierite-mullite materials up to 1300 ""C and SiC-based materials for higher temperatures. 

With a low thermal conductivity of 0.06 W cm-1 K 1 and a low thermal expansion coefficient a 4.5 x 10 6oC 1, mullite is useful for many refractory applications [49], According to Schneider, most mullites display low and nonlinear thermal expansions below, but larger and linear expansion above, 300°C. The volume thermal expansion... 

At the DLR Institute of Materials Research an all-oxide CMC (WHIPOX = wound highly porous oxide) consisting of alumina fibres Nextel 610 or mullite based fibre (Nextel 720, both 3M) and an alumina or mullite matrix, respectively, has been developed in recent years [1,2]. Mullite-based CMCs typically offer higher creep stability than alumina-based composites but display lower thermal conductivity. Lower thermal stability of alumina-based materials, however, can be accepted, since service temperature is significantly lower for these materials as a result of the better cooling efficiency. Therefore material development was focused on alumina... 

Various material properties (e. g. thermal diffiisivity, air permeability) have been determined for WHIPOX CMCs. Reliable data are important for potential applications such as thermal insulators, filters or burners. Thermal conductivity perpendicular to fiber orientation is about IW/mK. Closer inspection reveals lower conductivity if a mullite matrix is employed instead of alumina. Thermal conductivity in fiber direction, on the other hand, is about three times higher as perpendicular to the fiber direction, reflecting the non-isotropic structure of the composite (Figure 11). 

Conductive heat transfer has a phononic nature, which means that the heat is transferred due to the oscillation of the atoms in the crystal lattice. Crystals with a simple lattice, such as sihcon carbide or carbon, have a lower dissipation of heat waves and a higher thermal conductivity compared to crystals with a more complex lattice. For example, the conductive thermal conductivity of aluminium nitride or silicon carbide (binary compounds with approximately equal atomic weights) is higher than alumina, magnesia, and zirconia. And the conductive thermal conductivity of said alumina, magnesia, and zirconia is higher than that of spinel, mullite, and zircon. 

Fig. 1.17 Thermal conductivity of lightweight fireclay brick (7), fireclay brick (2), silica brick (5), mullite brick (4), alumina brick (5), zircon (6), cromite (5), magnesia (9) according to [85-87]...

One candidate material that meets most of these material requirements is mullite (3AI2O3-2 i02). Commercial mullites have thermal conductivities that are 5 to 6 times less than that of silicon nitrides which eliminates SijN4 as a candidate trtaterial because low thermal conductivity is cmcial to the design of the miniature turbine. Mullite also exhibits good high temperatures properties as it is... 

MuUite is another important crystalline ceramic that is an oxide It is a solid solution of alumina and silica in the compositional range 71-75 wt% alumina. Mullite is represented by the formula, 3 Al203.2Si02. It has excellent strength and creep resistance as well as low thermal expansion and conductivity. For more detailed information on the structure and properties of mullite the reader is referred to Schneider et al. (1994). Table 6.2 provides a summary of the properties of mullite. 

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