Hafnium diborides

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). 

Apart from the reactions described above for the formation of thin films of metals and compounds by the use of a solid source of the material, a very important industrial application of vapour phase transport involves the preparation of gas mixtures at room temperature which are then submitted to thermal decomposition in a high temperature furnace to produce a thin film at this temperature. Many of the molecular species and reactions which were considered earlier are used in this procedure, and so the conclusions which were drawn regarding choice and optimal performance apply again. For example, instead of using a solid source to prepare refractory compounds, as in the case of silicon carbide discussed above, a similar reaction has been used to prepare titanium boride coatings on silicon carbide and hafnium diboride coatings on carbon by means of a gaseous input to the deposition furnace (Choy and Derby, 1993) (Shinavski and Diefendorf, 1993). 

At elevated temperatures and in vapor phase, the tetrachloride reacts with air or steam forming finely divided hafnium dioxide, Hf02. When heated with boron trichloride and hydrogen to very high temperatures (above 2,000°C) hafnium diboride, H B2, a gray crystalline solid, forms. 

HfB2 hafnium diboride [12007-23-7] ">3250 TiB2 titanium diboride [12045-63-5] 2900 50... 

Zirconium and hafnium diboride have been studied less extensively. Zirconium diboride potentially is useful as a coating for solar absorbers [242]. These compositions can be prepared either by hydrogen co-reduction of the metal and boron halides [242], or by the decomposition of the metal tetrakis(tetrahydroborates) M(BH4)4 [241]. 

M. Gasch, D. Ellerby, E. liby, S. Beckman, M. Gusman, and S. Johnson, Processing, Properties and Arc Jet Oxidation of Hafnium Diboride/Silicon Carbide Ultra High Temperature Ceramics, J. Mater. Sci., 39, 5925-5937 (2004). 

Hafnium-like boron is known to be a neutron absorber or neutron moderator element, and, therefore, composites of boron carbide, B4C, and hafnium diboride, HfB2, can be considered as nuclear materials. These boron compounds after sintering and °B/"B isotopic ratio adapting are found to be heterogeneous polyphone cermets useful for nuclear applications (Beauvy et al. 1999). Boron acid obtained from the °B enriched boron trifluoride also was used in nuclear reactors (Shalamberidze et al. 2005). Amorphous boron powders enriched both in °B and "B, boron carbide, and zirconium diboride (ZrB2) powders and pallets labeled with °B isotope And applications in nuclear engineering too. The °B enriched Fe-B and Ni-B alloys are useful for the production of casks for spent nuclear fuel transfer and storage. 

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. 

Kaufman, L. and Clougherty, E. V. and Berkowitz-Mattuck, J. B. Oxidation Characterastics of Hafnium and Zirconium Diboride, Trans. TMS-AIME, [239] 458-466 (1967). 

Bargeron, C. B., Benson, R. C., Newman, R. W., Jette, A. N. and Phillips, T. E. Oxidation Mechanisms of Hafnium Carbide and Hafnium Diboride in the Temperature Range 1400 100C, Johns Hopkins APL Technical Digest, [14] 29-35 (1993). 

Gasch, M., Ellerby, D., Irby, E., Beckman, S., Gusman, M. and Johnson, S., Processing and Properties of Hafnium Diboride/Silicon Carbide Ultra High Temperature Ceramics, To be published in J. of Materials Science (2003). 

Hafnium Diboride (HfB2) 9.61 -H 10.72x10 cal/mole at 494-lOlOK... 

Borides Chromium Diboride (CrB2) Hafnium Diboride (HfB2) Tantalum Diboride (TaB2) ZlxlO 10-12 X 10 oaxio room temp. 

Hafnium Monocarbide (HfC) Tantalum Monocarbide (TaC) Zirconium Diboride (ZrB2) Zirconium Diboride (ZrB2) 0.053 at room temp. 0.053 at room temp. 0.055 0.058 at room temp. 0.055 0.060 at 200 °C... 

Titanium Diboride (TiB2) Hafnium Diboride (HfB2) (single crystal) Knoop 160g 3500 kg/mm Knoop 160g 3800kg/mm at 24 °C... 

Hafnium Diboride (HfB2) room temp. 10-12X10 ... 

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