Carbide thermal properties

Chromium carbide is important in powder preparations designed for thermal spray apphcations of corrosion and wear-resistant coatings on tool and machine parts. Lower carbon carbides of chromium are important in hardfacing tods and electrodes for weld-apphed ovedays on machine wear surfaces. However, these carbides are usually formed in situ from Cr and C in the rod and not added as preformed carbides. The properties of Ci2C2 are hsted in Table 2. 

There are, of course, many more ceramics available than those listed here alumina is available in many densities, silicon carbide in many qualities. As before, the structure-insensitive properties (density, modulus and melting point) depend little on quality -they do not vary by more than 10%. But the structure-sensitive properties (fracture toughness, modulus of rupture and some thermal properties including expansion) are much more variable. For these, it is essential to consult manufacturers data sheets or conduct your own tests. 

Improved stiffness, strength and thermal properties are offered by beta-Ti alloys reinforced with silicon carbide fibres. A high probability exists that these composites will make their way into the design of the actively cooled airframe of the future US NASP X30 spaceplane. 

There are a variety of advantages to using uranium nitride (UN) over the other fuel types, and a handful of drawbacks that need to be considered. Uranium nitride has a much higher thermal conductivity than uranium dioxide, resulting in a flatter temperature profile across the fuel pin. The same is likely true for uranium carbide, but less testing has been done on uranium carbide than for UN. Figure 3-3 shows the desired thermal properties of UN [Touloukian, 1979]. At the operational temperature, 1300 K, the thermal conductivity is 28.5 W/m-K and the linear expansion is roughly 1%. It is important for the different components to expand at similar rates to minimize the extra stresses. 

Ceramic borides, carbides and nitrides are characterized by high melting points, chemical inertness and relatively good oxidation resistance in extreme environments, such as conditions experienced during reentry. This family of ceramic materials has come to be known as Ultra High Temperature Ceramics (UHTCs). Some of the earliest work on UHTCs was conducted by the Air Force in the 1960 s and 1970 s. Since then, work has continued sporadically and has primarily been funded by NASA, the Navy and the Air Force. This article summarizes some of the early works, with a focus on hafnium diboride and zirconium diboride-based compositions. These works focused on identifying additives, such as SiC, to improve mechanical or thermal properties, and/or to improve oxidation resistance in extreme environments at temperatures greater than 2000°C. 

Hollow diamond fibers have been produced by using the CVD process to coat diamond onto a wire, preferably with a relatively low thermal expansion coefficient and having carbide-forming properties [9]. Wires of W, Ti, Ta or copper can be used or fibers of silica and silicon carbide, which are initially abraded with diamond grit to provide nucleation sites. The metals can be etched away in a suitable reagent such as nitric acid for Cu and hydrogen peroxide for W. The diamond deposit has a tensile modulus of about 880 GPa. 

Another benefit of the structural rigidity of many intermetallic compounds is that it prevents the dissolution of reactants underneath the surface. This prevents the formation of subsurface compounds, such as hydrides and carbides, which have been identified as being the catalytically active phase in elemental Pd hydrogenation catalysts as dealt within more detail below. Embedding the centers of reactivity in a dense atomic matrix like in the intermetallic compounds provides mechanical and structural stability and excellent thermal properties preventing the subsurface chemistry. 

Note Variations in the thermal properties of interstitial carbides are often found in the literature, reflecting the difficulty of these measurements at such high temperatures and the essentially non-stoichimetric nature of interstitial carbides. The values given here are a general average. 

Table 4.2 Thermal Properties of Group IV Interstitial Carbides and Other Refractory Materials...

The thermal properties of the covalent carbides are shown in Table 8.4.[151[16]... 

Table 8.4 Thermal Properties of Covalent Carbides and Other Refractory Materials at 20 C...

Turkes, P. R., Swartz, E. T., and Pohl, R O., Thermal Properties of Boron and Boron Carbide, in Boron-Rich Solids, AIP Conf. Proc. 140 (D. Aselage et al., eds.), Am Inst, of Physics, New York (1986)... 

Morgan Advanced Ceramics, Thermal Properties of Silicon Carbide. 

Worrell and Chipman (1964) measured the CO pressure over a mixture of Nb02, C and NbC between 1170° and 1260°. The resulting heat of formation, shown in Fig. 27, is raised by 0.3 kcal if the thermal properties in Table 29 are used. Since oxygen was present, this measurement applies to the oxycarbide although, in this system, the difference between the carbide and the oxycarbide would appear to be slight. 

K. Schmidt and C. Zweben, Mechanical and Thermal Properties of Silicon Carbide... 

Composites of silicon carbide (SiC) and silicon (Si) are fabricated by the reactive infiltration of molten Si into preforms of SiC particles and carbon. This product is often referred to as reaction bonded silicon carbide (RBSC). SiC materials are used in many applications due to their favorable properties including high hardness, high thermal conductivity, low thermal expansion and high stiffness. This paper demonstrates the manipulation of thermal and mechanical properties through the additions of third phase metals (e.g. A1 and/or Ti) to the infiltration alloy and through the additions of ceramic-forming, reactive materials to the preform. The effects of these additions on microstructural, physical, mechanical, and thermal properties are presented and discussed. 

These working conditions led to the selection of non-oxide refractory ceramics as fuel coating. Thus, carbides turn out to be great candidates thanks to their remarkable mechanical and thermal properties. However, their behaviour under irradiation has to be studied in more details. 

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