Borides thermal conductivities

The refractory-metal borides have a structure which is dominated by the boron configuration. This clearly favors the metallic properties, such as high electrical and thermal conductivities and high hardness. Chemical stability, which is related to the electronic... 

The problems associated with direct reaction calorimetry are mainly associated with (1) the temperature at which reaction can occur (2) reaction of the sample with its surroundings and (3) the rate of reaction which usually takes place in an uncontrolled matmer. For low melting elements such as Zn, Pb, etc., reaction may take place quite readily below S00°C. Therefore, the materials used to construct the calorimeter are not subjected to particularly high temperatures and it is easy to select a suitably non-reactive metal to encase the sample. However, for materials such as carbides, borides and many intermetallic compounds these temperatures are insufficient to instigate reaction between the components of the compound and the materials of construction must be able to withstand high temperatures. It seems simple to construct the calorimeter from some refractory material. However, problems may arise if its thermal conductivity is very low. It is then difficult to control the heat flow within the calorimeter if some form of adiabatic or isothermal condition needs to be maintained, which is further exacerbated if the reaction rates are fast. 

Room temperature resistivities of YB and GdB66 take values of 3 x 102 and 5 x 102 12 cm, respectively (Golikova, 1987) and the RB66 phases can be considered as insulators. The characteristic temperature of the VRH To for example for GdB66 was determined as 4 x 107 K. The conductivity is p-type. The thermal conductivity and thermopower of RB66, will be discussed in Section 12 on "Thermoelectrics in higher borides". 

However, carbon doping was found to decrease the thermal conductivity of YB66 while not having a sizable detrimental effect on the other properties, and this could be a powerful method for improving the thermoelectric properties of higher borides in general (Mori and Tanaka, 2006). 

From Eq. (11), an obviously desirable characteristic for thermoelectric materials is to have low thermal conductivity k. The thermal diffusivity constant, Dt, of ErB44Si2 has been found to have small values of Dt < 1.1 x 10 2 cm 2/s (Mori, 2006c). These values are significantly smaller than what has been observed for boron carbide samples (Wood et ah, 1985). Although no data exists for the sound velocities of ErB44Si2, the velocities are probably high since borides are typically hard materials. Therefore, the small values of Dt indicate extremely short phonon... 

Intense research has in recent years been devoted to noncrystalline materials. It was discovered also that the majority of semiconducting boron-rich borides display several properties that resemble those of the noncrystalline solids. Among the amorphous properties are the temperature and field dependencies of electrical conductivity at low temperature, the temperature dependence of thermal conductivity at high temperatures, and the temperature dependence of the magnetic susceptibility. In addition, the boron-rich semiconductors display crystalline properties, for example, the temperature dependence of the thermal condnctivity at low temperatures, the lattice absorption spectra and the possibility to change... 

Figure 6. Temperature dependence of thermal conductivity of arc-melted silicon borides before and after heat-treatment at 1673Kfor 0.5hr.

Beauvy, M., D. Stai u, P. Provot, and D. Gosset. 1999. Thermal conductivity of heterogeneous boride careers for nuclear uses Influence of microstructure and estimation. In Abstracts of the 13th International Symposium on Boron, Borides and Related Compounds, Dinard, France, p. 45. 

Transition metal borides are mainly explored for their mechanical properties. Since they exhibit metallic transport properties such as high electric and thermal conductivity with a negative temperature coefficient they are also of interest as electrode materials, for heating elements and sensors. 

The poor corrosion resistance of borides may be partially overcome in composites with SiC if the microstructure allows passive oxidation kinetics. In combination with SiC the borides retain their high electrical and thermal conductivity and thus suitable thermal shock resistance. SiC-TiB2 composites have been extensively developed for wear parts in machinery such as sliding rings, valves, valve seats, roller and ball bearings, plungers, and rocker arm pads. 

As previously indicated, the thermal conductivities of borides are typically high, in comparison to many other ceramics and are a result of both a lattice and an electronic contribution to phonon transport. Figure 8 illustrates the large difference in conductivities of... 

Of note are the higher specific heats and thermal conductivities of the borides. 

As can be seen in Table 4.2, the thermal conductivities of the Group rv carbides, nitrides, and borides are relatively close. They are also similar to those of the host metals and, from this standpoint, reflect the metallic character of these compounds. However, their conductivities are much lower than that of the best conductors such as Type II diamond (2000 W/m-K), silver (420 W/m-K), copper (385 W/m K), beryllium oxide (260 W/m-K), and aluminum nitride (220 W/m-K). 

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