Title

Synthesis and characterization of group IV and V metal diboride nanocrystals via borothermal reduction of metal oxide with NaBH4

Conference Dates

September 17-20, 2017

Abstract

Group IV and V metal diborides (MB2) have a unique combination of properties such as a very high melting point (>3000°C), high hardness, good solid-state phase stability, high thermal and electrical conductivity. Metal diboride-based ceramics are expected to be potential candidate materials for ultra-high-temperature applications in the aerospace industry [1]. Due to the poor sinterability of commercial powders, the availability of nanometric boride particles has indeed the potential to improve several stages of ceramic processing [2], or for instance to facilitate the sintering of bulk ceramics due to enhanced particle reactivity [3]. Several synthesis have been developed to achieve nanoborides: chemical route from inorganic precursors, mechanical alloying and self-propagating high-temperature synthesis [4–6].

In this work we proposed the synthesis of group IV and V metal diboride (MB2, M= Ti, Zr, Hf, Nb, Ta) nanocrystals by a thermal treatment of the metal oxide and sodium borohydride (NaBH4) at 700°C under atmospheric pressure [7]. The reaction occurs first via decomposition of NaBH4, followed by the formation of amorphous boron and crystalline ternary species with general formula NaxMyOz and NaxByOz. Finally all of the intermediary species yield metal diboride (MB2) and sodium meta-borate (NaBO2).

Synthesized TiB2 nanocrystals have an average size of 11 nm and the powder has a specific surface area (s.s.a) of 33.45 m2/g. ZrB2 grains have a platelet morphology with an aspect ratio of 10, average size of 22.5 nm and s.s.a of 24.97 m2/g; HfB2 has a similar morphology with a crystals size of 28 nm, while the s.s.a is even higher, 36.36 m2/g. As far as we know, the latter is the finest powder obtained via borothermal reduction of metal oxides ever reported. Synthesized NbB2 powder consists of crystallites around 12 nm and has a s.s.a of 21.09 m2/g. TaB2 powder has a s.s.a of 11.38 m2/g and consists of 200 nm agglomerates of spherical and needle-shaped nanocrystals with average size of 11 nm.

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29 - Luca Zoli.pdf (188 kB)

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