Ceramics thermal conductivity values

Thermal conductivity values for ceramics, refractory oxides, and miscellaneous insulating materials are given here. The thermal conductivity refers to samples with density indicated in the second column. Since most of these materials are highly variable, the values should only be considered as a rough guide. 

Thermal conductivity values of polymers are lower than those of metals and ceramics in table 1 the range of thermal conductivity is of the order of 0.1-0.5 W/m.K. As a general rule, crystalline polymers have higher conductivities than amorphous polymers [7], Price et al find that the thermal conductivity of semi-crystalline polymers (PTEF) tend to increase linearly with crystallinity at 232°C (Figure 1) [8],... 

Alumina porcelains contain corundum and glass phases. Both phases are continuous at a composition corresponding to about 9 vol% glass. In such a case, the thermal conductivity value will be in between that of the two phases. Generally, the glass is continuous in vitreous ceramics. Therefore, the conductivity of these materials is closer to that of the glass phase. 

Thermal conductivity values for a number of ceramic materials are given in Table 19.1 room-temperature thermal conductivities range between approximately 2 and 50 W/m K. Glass and other amorphous ceramics have lower conductivities than crystalline ceramics because the phonon scattering is much more effective when the atomie structure is highly disordered and irregular. 

Ceramic materials are lightweight and have low thermal conductivity. However, at elevated operating temperatures, radiation contributes significantly to the total heat transfer through the ceramic foam, because most ceramics are partially transparent to radiation Thus, the heat transfer would be higher than expected based on ceramic foam s room temperature thermal conductivity alone. Cochran et at concluded that muliite and alumina foams made from spheres had low thermal conductivity values at low temperatures, see Reference 9- 80 Additional potential problems include material compatibility issues and mechanical properties... 

The value of the coefficient will depend on the mechanism by which heat is transferred, on the fluid dynamics of both the heated and the cooled fluids, on the properties of the materials through which the heat must pass, and on the geometry of the fluid paths. In solids, heat is normally transferred by conduction some materials such as metals have a high thermal conductivity, whilst others such as ceramics have a low conductivity. Transparent solids like glass also transmit radiant energy particularly in the visible part of the spectrum. 

InN single crystals of a size suitable for thermal conductivity measurements have not been obtained. The only measurement of the thermal conductivity has been made using InN ceramics [20], InN microcrystals obtained by microwave plasma were sintered under a pressure of 70 kbar at 700°C. The room temperature thermal conductivity was measured by the laser-flash method giving k = 0.45 W/(cm K) [20], This value is much below the estimate by Slack which gives k = 0.8 W/(cm K). This result indicates that the InN ceramic has a high impurity content and consists of small size grains. 

Silicon carbide, widely employed as an abrasive (carborundum), is finding increasing use as a refractory. It has a better thermal conductivity at high temperatures than any other ceramic and is very resistant to abrasion and corrosion especially when bonded with silicon nitride. Hot-pressed, self-bonded SiC may be suitable as a container for the fuel elements in high-temperature gas-cooled reactors and also for the structural parts of the reactors. Boron carbide, which is even harder than silicon carbide, is now readily available commercially because of its value as a radiation shield, and is being increasingly used as an abrasive. 

There are two possible approaches to the selection of materials from the standpoint of thermal shock resistance. The first is suitable for glass and fine dense ceramics and was discussed in Section II. 5. 2. With these materials, it is necessary to avoid formation of primary cracks which originate at the surface and propagate rapidly into the interior where they are the cause of extensive fracture. In this case, the favourable properties include high strength and high thermal conductivity, and low elasticity modulus and expansion coefficient values. 

Insulating materials, such as the ceramics, have very low electrical conductivity because they have essentially no free electrons to carry current. There are no electrons in the conduction band, and the band gap is too large for electrons to be promoted from the valence band to the conduction band. Semiconductors have conductivity values intermediate between metals and insulators because their bandgaps are small enough that electrons can be promoted from the valence band to the conduction band with modest thermal or optical excitation. 

Fig. 10 Schematic of temperature distributions in a flat ceramic slab of thickness L for volumetric microwave heating (top curve) and conventional heating from the slab surfaces (bottom curve). For conventional heating, the finite value of thermal conductivity, k, gives the highest temperatures near the specimen surface and the lowest temperature along the specimen s midplane. Conversely, for microwave heating the heating is more uniform, with decreasing temperature near the slab surface because of heat losses from the surfaces.

The Seebeck coefficient a and figure of merit Z for B4C-B ceramics as a function of C content are given in Fig. 7 and Fig. 8, respectively. The a was always positive, and its absolute value increases with increasing carbon content except for B4C+5B. a (0.30 0.38 mV/K), whose maxima was observed at 20 at.% C (B4O sintered at 2250 °C, showed opposite tendency of electric resistivity (7X 10 6 X10" Qm), whose minimum was at B4C+8B sample fired at 2250°C. Though the figure of merit Z is evaluated from electrical resistivity, thermal conductivity and Seebeck coefficient, the Z values showed maximum of 2.4 X 10 K at B4C+8B composite fired at 2250 °C. Therefore, the electric resistivity affects more than the Seebeck coefficient. 

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