Hafnium nitride, properties

Most hafnium compounds have been of slight commercial interest aside from intermediates in the production of hafnium metal. However, hafnium oxide, hafnium carbide, and hafnium nitride are quite refractory and have received considerable study as the most refractory compounds of the Group 4 (IVB) elements. Physical properties of some of the hafnium compounds are shown in Table 4. 

Hafnium nitride (HfN) is a refractory which is resistant to chemical attack. It is produced by CVD mostly on an experimental basis. Its properties and characteristics are summarized in Table 10.4. 

The cubic phases of zirconium- and hafnium nitride display properties quite similar to those of TiN. These compositions have been studied to some extent, however, the number of reports dealing with the CVD of ZrN and HfN is considerably lower than those describing the CVD of TiN. Zirconium and hafnium nitride can be prepared by the decomposition of the appropriate metal tetrakis(dialkylamide) compounds [155]. Zirconium and hafnium nitrides of the stoichiometry M3N4 also have been prepared by CVD. The compounds are formed when M(NEt2)4 is reacted with NH3 at 200-450°C [144]. The films thus produced are crystalline, yellow, transparent and insulating. It has been suggested that these phases are related to the MN phase by a rhombohedral distortion. 

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. 

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