Novosibirsk, Russia 2 Budker Institute of Nuclear Physics, SB RAS, , Novosibirsk,

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1 MANUFACTURE OF PRODUCTS FROM MOLTEN HAFNIUM CARBIDE BY MEANS OF THE TREATMENT OF MECHANOCOMPOSITE WITH A HIGH-INTENSITY FOTON FLUX A.I. Ancharov 1, 2, T.F. Grigoreva 1, G.N. Grachev 3 1 Institute of Solid State Chemistry and Mechanochemistry, SB RAS, Novosibirsk, Russia ancharov@mail.ru 2 Budker Institute of Nuclear Physics, SB RAS, , Novosibirsk, Russia 3 Institute of Laser Physics, SB RAS, , Novosibirsk, Russia The mechanocomposites hafnium/carbon was performed in a watercooled high-energy planetary ball mill during 2 min. Molten hafnium carbide from the mechanocomposite hafnium/carbon was obtained under the action of high-intensity laser radiation. High-resolution XRD studies of the samples were carried out using hard (quantum energy of 33.7 kev) synchrotron radiation. The study of morphological characteristics of the samples was carried out using scanning electron microscope. Introduction Hafnium carbide is known at present as the material with the highest melting points. According to the most recent data, the melting point of hafnium carbide was determined to be 3965 C ±50 C, tantalum carbide 3768 C ±40 C, tantalum-hafnium carbide Ta 4HfC C ±40 C [1]. In addition, they are among the hardest materials. The development of modern technology requires the products made of the materials able to endure high temperatures and possessing high stability to oxidation at these temperatures. In addition to the high melting point, hafnium carbide is characterized by high hardness, chemical stability, low pressure vapor, good thermoshock resistance; these characteristics are necessary for the materials to be used in aerospace technologies. Hafnium carbide possesses low electron work function, which is 2.04 ev. For comparison, lanthanum hexaboride, which is widely used to fabricate cathodes, has the electron work function 2.66 ev and melting point 2740 C. So, the most efficient cathodes for charged-particle accelerators may be made of hafnium carbide. 4

2 The products made of hafnium carbide are manufactured by means of powder metallurgy. Carbides are characterized by a poor compressibility because of their high hardness, so the products obtained after sintering are highly porous. To decrease the porosity, it is possible to use hot pressing techniques [2, 3] and spark plasma [4, 5]. At present, the majority of works deal with obtaining hafnium carbide in the fine state, which would allow one to decrease porosity and improve powder sintering. The goal of the work was to study the processes that would allow obtaining the products made of hafnium carbide by means of melting. Experimental Powders of hafnium and lamp black in the stoichiometric ratio were used to prepare hafnium carbide. Mechanochemical synthesis of the hafnium carbide was performed in an AGO-2 water-cooled high-energy planetary ball mill in an argon atmosphere. The milling drum volume was 250 cm 3, the ball mass was 200 g, the sample mass was 10 g, and the rate of drum rotation around the common axis was ~ 1000 rpm. High-resolution XRD studies of the samples were carried out using hard (quantum energy of 33.7 kev) synchrotron radiation (SR) at the 4th SR beamline of the VEPP-3 storage ring at the Siberian Synchrotron and Terahertz Radiation Centre [6]. The powdered samples were placed in a thin layer in the storage ring and were analyzed in transmission geometry. The primary beam size was mm. The diffracted radiation was recorded by a mar345 X Y recorder. The exposure time was 10 min. The data obtained by the X Y recorder were integrated over all directions. XRD patterns were used for phase analysis of the samples. The tested precision of the instrument in determining the interplanar distances using standard samples was no lower than Å. The study of morphological characteristics of the samples was carried out using scanning electron microscope (Oxford Instruments and Hitachi TM 1000) at an accelerating voltage of 15 kv. 5

3 Results and discussion The mechanical activation of the Hf + C mixture (50:50 at.%) for 2 min leads to the formation of the Hf/C mechanocomposite; see XRD patterns (Fig. 1, curves 2, 3) showing broadened hafnium reflections. Fig. 1. X-ray diffraction patterns of the initial Hf (1) and the Hf + C mixtures mechanically activated for 10 s (2), 2 min (3). To solve the formulated problem, we used the treatment of the mechanocomposite of hafnium and carbon by high-intensity fluxes of electrons and photons. High-intensity photon and electron fluxes allow rapid heating of a limited volume of the sample to a high temperature (above 6000 C). So high temperature allows one not only to initiate the formation of hafnium carbide but also to melt it and even to bring it to boiling. Preliminary experiments aimed at obtaining molten hafnium carbide from the mechanocomposite of hafnium and carbon were carried out under the action of high-intensity laser radiation. A weighted portion of mechanocomposite was put in a graphite crucible with the inner diameter of 9 mm and tightened by pressing. The sample was moved over 6

4 a two-axis table so that the laser beam irradiated the sample along a spiral converging to the center. Two experiments were carried out. In the first case, hafnium carbide powder was used; in the second case, the mechanocomposite of hafnium with carbon was used. One can see in Fig. 2 that the formation of the melt of hafnium carbide starts at first in the mechanocomposite, and later in hafnium carbide powder. X-ray studies showed that similar diffraction patterns coinciding with the reference were recorded in both cases. Fig. 2. The samples of fused hafnium carbide obtained from hafnium carbide powder (left) and from the mechanocomposite of hafnium and carbon (right). The thickness of the layer of fused hafnium carbide on the sintered layer is about 0.3 mm. It may be concluded relying on the above-mentioned data that the powders of the mechanocomposite of hafnium and carbon are preferable as the material for obtaining the products made of fused hafnium carbide by means of additive technologies. This research was carried out within the Integrated Program of the Siberian Branch of RAS Integration and Development (projects 73.3). References 1. Cedillos-Barraza O, Manara D, Boboridis K, et. al.: Investigating the highest melting temperature materials: A laser melting study of the TaC-HfC system. Scientific reports

5 2. Ramqvist L: Hot pressing of metallic carbides. Powder Metallurgy (17) Zhang X, Hilmas G E, Fahrenholtz W G, Deason D M. Hot pressing of tantalum carbide with and without sintering additives. Journal of the American Ceramic Society (2) Sciti D, Guicciardi S, Nygren M. Densification and mechanical behavior of HfC and HfB 2 fabricated by spark plasma sintering. Journal of the American Ceramic Society (5) Ghaffari S A, Faghihi-Sani M A, Golestani-Fard F, Mandal H. Spark plasma sintering of TaC HfC UHTC via disilicides sintering aids. Journal of the European Ceramic Society (8) Ancharov A, Manakov A, Mezentsev N, Sheromov M, Tolochko B, Tsukanov V: New station at 4th beamline of the VEPP-3 storage ring. Nuclear Instruments and Methods in Physics Research 2001 A