Hafnium boride

Hafnium Boride. Hafnium diboride [12007-23-7] HfB2, is a gray crystalline soHd. It is usually prepared by the reaction of hafnium oxide with carbon and either boron oxide or boron carbide, but it can also be prepared from mixtures of hafnium tetrachloride, boron trichloride, and hydrogen above 2000°C, or by direct synthesis from the elements. Hafnium diboride is attacked by hydrofluoric acid but is resistant to nearly all other reagents at room temperature. Hafnium dodecaboride [32342-52-2] has been prepared by direct synthesis from the elements (56). 

In finely divided form, hafnium is pyrophoric, igniting in air spontaneously. However, bulk metal reacts slowly in oxygen or air above 400°C. The rate of oxidation increases with temperature. The product is hafnium dioxide, Hf02. It combines with nitrogen, carbon, boron, sulfur and silicon at very high temperatures to form hafnium nitride HfN, hafnium boride HfB, hafnium sulfide HfSi2, respectively. Nitride formation occurs at 900°C. 

Fig. 18. The Hf-B-C isothermal ternary cross sections at three different temperatures. Points A and B represent two possible stoichiometries for processing hafnium boride/hafnium carbide composites. After Rudy [57],...

Chromium boride Hafnium boride Molybdenum boride... 

Figure 6.13. The calculated stability diagram at 1125°C of a mixture of hafnium chloride, boron chloride, and methyl trichlorosilane. The solids expected to be in equilibrium with a gas of the stated composition are noted in the different concentration ranges. The composition of the gas mixture at which both silicon carbide and hafnium boride are stable is shaded. From W. J. Lackey, A. W. Smith, D. M. Dillard and D. J. Twait. Proc. 10th Intern. Conf. CVD, Honolulu (1987), p. 1008. Reproduced by permission of The Electrochemical Society, Inc.

Potentially useful single crystal HP-LCVD fibers include hafnium boride and tantalum carbide and have projected service temperatures ranging from 2170 to 2715 C. Presently envisioned applications include the potential use of these fibers as consumable sensors to monitor rocket exhaust temperatures. Other HP-LCVD sensor fibers, including Si, Ge and ZnSe, (Figure 15), promise to offer high value in premium automotive and medical sensor systems. Single crystal HP-LCVD germanium [20] and silicon carbide [21] fibers can now also become available for exploration. In summary, the HP-LCVD process is an ideally suited tool for the rapid fabrication and evaluation, without extensive process research, of test samples of potentially new fiber candidates for structural and sensor uses. 

Reich, S., Hnrald, S., Klhra, H., Laszlri, S. (1992). Deposition of thin films of zirconium and hafnium boride by plasma enhanced chemical vapor deposition. Advanced Materials, 4(10), 650-653. doi 10.1002/adma. 19920041005. 

---