Mechanically activated solid-state synthesis of hafnium carbide and hafnium nitride nanoparticles

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Abstract

Nanocrystalline hafnium nitrides and hafnium carbides are synthesized, from powder mixtures based on partially hydrated hafnium tetrachloride, ph-HfCl4, and on magnesium, by a three-step process, namely a short mechanical activation step followed by a brief annealing step and a leaching step. Grinding of ph-HfCl4 + Mg leads to the formation of hafnium hydrides which decompose upon annealing in argon to give first metallic hafnium and then HfO2 at higher temperatures. Annealing of such ground ph-HfCl4 + Mg blends in flowing nitrogen yields HfN nanoparticles (average size between 10 and 30 nm). HfC carbides are directly mechanosynthesized from ph-HfCl4 + Mg + C powder mixtures. A subsequent heat treatment in flowing argon allows to better crystallize the grains of HfC (average size around 100 nm) and to decompose the intermediate products. The reaction mechanisms are discussed from results of thermogravimetric analyses and from infrared spectra of ph-HfCl4 + Mg based powder mixtures processed in various ways.

Introduction

Research activities in the field of nanostructured materials are largely devoted to the development of simple, fast, and cheap synthesis methods. Among the currently investigated methods, reactive ball-milling in which reactions occur during milling and mechanical activation, where milling is instead used to change the reactivities of solids, appear to be appealing methods for preparing material precursors [1], [2], [3], [4], [5]. The usual conditions for which chemical reactions take place in powders may indeed be changed by an intermediate step of high-energy ball-milling which favors the mixing of elements at the nanometer scale and produces defects. As a result, the final products are most often formed at lower temperatures and at higher rates than in usual conditions. For instance, boron nitride nanotubes were prepared by annealing, under nitrogen, boron powders ground in NH3 [6]. Similarly, nanorods of HfB2 were produced by annealing of ground HfCl4 + B powder mixtures [7] and nanosized CeO2 particles were obtained after annealing of mechanically activated powders of cerium carbonate with sodium hydroxide [8]. Many studies are, however, based on mechanically activated exchange reactions between, for example, metallic chlorides and MOH (with M = Li or Na) which lead to nanosized monocrystalline particles embedded within a salt matrix which is further removed by washing [9], or between metallic chlorides and CaO [10] or Li2O [11] or between lithium and cobalt hydroxides [12]. The final product is most often an oxide.

The purpose of the present work is to synthesize nanostructured hafnium nitrides and carbides from partially hydrated hafnium tetrachloride, denoted hereafter as ph-HfCl4, by short steps of mechanical activation which prove to be methods advantageous to prepare, in mild milling conditions, nanoparticles which are not agglomerated. Nanostructured polycrystalline powders of metal carbides are usually synthesized by direct grinding of metallic powders with graphite [13] or with organic compounds like heptane [14] or of powder mixtures containing a metallic oxide, carbon and a strong reducing agent such as Mg, Ca and Al [15]. When a mild reducing agent is used, an additional heat treatment is necessary to produce carbides. Nitrides are similarly obtained by grinding metallic powders under a N2 [16] or a NH3 [17] atmosphere or with organic compounds containing nitrogen atoms [18] and by grinding metallic oxides with a strong reducing agent under N2 [19]. Hafnium nitrides and carbides are unique materials with high melting points, high hardnesses, used in the fields of wear resistance tools, cutting tools, reinforcement of ceramics and metals, and ultra-high temperature structural applications [20], [21], [22]. The high melting points of these materials and the high stability of the oxide phase make it difficult to produce them notably in a monocrystalline form. Moreover the starting product, partially hydrated HfCl4, is a by-product of zirconium industry, stored in conditions in which it becomes hydrated, which appears to be useful to produce high refractory materials.

We shall report in this article on the reduction of partially hydrated HfCl4 by magnesium and on the synthesis of hafnium nitrides and hafnium carbides by a three-step process: mechanical activation (∼1–2 h) followed by annealing (∼1 h) of ph-HfCl4 + Mg based powder mixtures and leaching. Magnesium was chosen because of its strong reducing capacity and of its expected potentiality to bypass the formation of the stable hafnium oxide. Moreover, it is insoluble in metallic hafnium and the unsought reaction products (MgCl2·nH2O, Mg or MgO) may be eliminated during the additional heat treatment or by further leaching. Characterizations of powders by X-ray diffraction, thermogravimetric analysis, scanning and transmission electron microscopy, Fourier transform infrared spectroscopy were performed to follow the phase evolution and to clarify the reaction mechanisms.

Section snippets

Experimental details

The minimum amounts of C (99.9% pure, average particle size ∼325 meshs, from CERAC) and Mg (99% pure, average particle size ∼300 μm, from Aldrich) in initial powder mixtures were obtained from the three following reactions: (i) ph-HfCl4 + 2Mg  Hf + 2MgCl2; (ii) ph-HfCl4 + 2Mg + C  HfC + 2MgCl2; ph-HfCl4 + 2Mg + 1/2N2  HfN + 2MgCl2. The reactants (Mg and/or C) were added in excess (up to 100 wt.%) to the previous stoichiometric contents. To form nitrides, nitrogen (99.9% pure, from Air Liquide) was introduced during

Results and discussion

As explained above (Section 1), the purpose of the present work is to prepare hafnium carbides and hafnium nitrides nanoparticles by successive steps, namely, a short activation of powder mixtures by ball-milling (∼1–2 h), followed by annealing (∼1–2 h) and by leaching. We report first on the study of the structural evolution of ph-HfCl4 + Mg mixtures after grinding and annealing in flowing argon whose aim was to clarify their transformation mechanisms. The next parts are then devoted to the

Conclusion

Mechanical activation of ph-HfCl4 + Mg powder mixtures followed by annealing in flowing nitrogen yields hafnium nitrides with nanometer-sized grains. Both steps are of short duration. The intermediate products of the reaction, namely hafnium hydrides, prevent hafnium from being oxidized and lead to a full nitridation reaction of Hf into HfN. Grinding of the same mixture with carbon leads to the direct formation of hafnium carbides. During annealing treatments, anhydrous MgCl2 is evaporated and

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