Combustion nitrides

Figure 1.18 Silicon infiltration combustion nitriding completeness degree ( f) on dilution...

Materials made of siHcon nitride, siHcon oxynitride, or sialon-bonded siHcon carbide have high thermal shock and corrosion resistance and may be used for pump parts, acid spray nozzles, and in aluminum reduction ceUs (156—159). A very porous siHcon carbide foam has been considered for surface combustion burner plates and filter media. It can also be used as a substrate carrying materials such as boron nitride as planar diffusion source for semiconductor doping appHcations. 

When you pour boiling water into a cold bottle and discover that the bottom drops out with a smart pop, you have re-invented the standard test for thermal shock resistance. Fracture caused by sudden changes in temperature is a problem with ceramics. But while some (like ordinary glass) will only take a temperature "shock" of 80°C before they break, others (like silicon nitride) will stand a sudden change of 500°C, and this is enough to fit them for use in environments as violent as an internal combustion engine. 

These nitridation reactions are also characterized by very high values of activation energy. As a result, the nitridation front represents a strongly nonlinear combustion wave which may be extremely thin and corrugated. 

Titanium is regarded by many authors as the only element that burns in nitrogen where it changes into nitride. This is not exactly correct since the same goes for lithium. Sometimes the combustion is capable of detonation. 

It is with the understanding of the above that one can give some insight to what establishes the pyrophoricity of small metal particles. The term pyrophoricity should pertain to the instantaneous combustibility of fine metal particles that have no oxide coat. This coating prevention is achieved by keeping the particles formed and stored in an inert atmosphere such as argon. Nitrogen is not used because nitrides can be formed. When exposed to air, the fine metal particle cloud instantaneously bursts into a flame. Thus it has been proposed... 

In the field of high-temperature combustion synthesis, metals have been reacted with nitrogen, both in the gaseous and liquid phases, to form refractory nitrides [2], In most cases, this nitriding process is heterogeneous. 

One of the simplest calorimetric methods is combustion bomb calorimetry . In essence this involves the direct reaction of a sample material and a gas, such as O or F, within a sealed container and the measurement of the heat which is produced by the reaction. As the heat involved can be very large, and the rate of reaction very fast, the reaction may be explosive, hence the term combustion bomb . The calorimeter must be calibrated so that heat absorbed by the calorimeter is well characterised and the heat necessary to initiate reaction taken into account. The technique has no constraints concerning adiabatic or isothermal conditions hut is severely limited if the amount of reactants are small and/or the heat evolved is small. It is also not particularly suitable for intermetallic compounds where combustion is not part of the process during its formation. Its main use is in materials thermochemistry where it has been used in the determination of enthalpies of formation of carbides, borides, nitrides, etc. 

The metals also present an additional problem in that the product oxide, fluoride, or nitride species may be a solid phase at the combustion temperature or condense during expansion through the rocket nozzle. Figure 2 presents a graphical comparison of the phase properties of the... 

The synthesis of titanium nitride by a combustion method was first studied by Moissan in 1892, as reported by Hlavacek.27 Moissan found that titanium has a strong affinity for nitrogen and burns in an atmosphere of gas at 1070 K with incandescence forming the hard, bronze-colored mononitride, TiN. 

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