Crystallographic Study of Product Phases of Carbothermic Reduction and Nitridation of Hafnium Dioxide

Details of the carbothermic reduction/nitridation tosynthesizehafnium nitride (HfN) and hafnium carbide (HfC) are scarce in theliterature. Therefore, this current study was carried out to evaluatetwo pathways for synthesizing these two refractory materials: directnitridation and carbothermic reduction/nitridation. Two mixtures ofhafnium dioxide and carbon with C/HfO2 molar ratios of2.15 and 3.1 were nitridized directly using flowing nitrogen gas atelevated temperatures (1300-1700 & DEG;C). The 3.1 C/HfO2 molar ratio mixture was also carbothermically reduced underflowing argon gas to synthesize HfC, which was converted into HfNby introducing a nitridation step under both N-2(g) andN(2(g))-10% H-2(g). X-ray diffraction results showedthe formation of HfN at 1300 and 1400 & DEG;C and HfC1-y N y at & GE;1400 & DEG;Cunder direct nitridation of samples using a C/HfO2 molarratio of 2.15. These phase analysis data together with lower latticestrain and greater crystallite sizes of HfC1-y N y that formed at higher temperaturessuggested that the HfC1-y N y phase is preferred over HfN at those temperatures.Carbothermic reduction of 3.1 C/HfO2 molar ratio samplesunder an inert atmosphere produced single-phased HfC with no significantlevels of dissolved oxygen. Carbothermic reduction nitridation madetwo phases of different carbon levels (HfC1-y N y and HfC1-y & PRIME;N y & PRIME;, wherey & PRIME; < y), while direct nitridation produceda single HfC1-y N y phase under both N-2 and N-2-10% H-2 cover gas environments. Tworeaction routes, namely, direct nitridation and carbothermicnitridation of a mixture of hafnium dioxide and carbon, were usedin the synthesis of hafnium nitrides. Hafnium carbide (HfC) was usedas the intermediate in the carbothermic nitridation route. Crystallographiccharacterization indicated that direct nitridation produced HfN attemperatures & LE;1400 & DEG;C, while hafnium carbonitride [Hf(C,N)] was produced by both routes at temperatures & GE;1400 & DEG;C.