Preparation of nano-size ZrB2 powder by self-propagating high-temperature synthesis

https://doi.org/10.1016/j.jeurceramsoc.2008.09.006Get rights and content

Abstract

Preparation of nano-size ZrB2 powder by SHS has been investigated. Zr and B elemental powders were mixed with 10–50 wt.% NaCl, and prepared pellets were reacted under argon. Adiabatic temperatures were calculated by HSC software. Increasing NaCl content led to a continuous decrease in adiabatic temperatures and reaction wave velocity. Products were subjected to XRD, SEM and FESEM analyses. Average crystallite size of ZrB2, which was 303 nm without NaCl, decreased to 32 nm with 40% NaCl addition. Distinct decrease in ZrB2 particle size was also observed from SEM analyses. 30% NaCl addition was found to be optimum for ensuring a stable SHS reaction and providing the formation of nano-size ZrB2 particles. It was revealed from particle size distribution measurements that ZrB2 powder obtained by 30 wt.% NaCl addition contained particles mostly finer than 200 nm. A mechanism, similar to solution-precipitation was proposed for the particle size refining effect of NaCl.

Introduction

Zirconium diboride (ZrB2) is one of the most stable borides.1 It is in ultrahigh-temperature ceramics class with a melting point of as high as 3050 °C. It has outstanding wear and corrosion resistance, high heat and electrical conductivity, and high hardness.2 Possible applications of ZrB2 bearing ceramics involve cutting tools, crucibles for molten metal handling, high-temperature electrodes and high-temperature spray nozzles.1, 2, 3 ZrB2 has been produced through various methods starting from elemental Zr or its oxide, ZrO2. Reaction between Zr and B elemental powders, metallothermic reduction of ZrO2 and B2O3 by magnesium or boron,4, 5 fused salt electrolysis,6 mechanochemical synthesis and combustion synthesis7 are some of the methods.

In the last decade, there has been a growing appeal on the production of ceramic powder having ultrafine or nano-sized particles and nano-grained sintered particles. Exceptional properties such as excellent sinterability of nano-powder and improved mechanical properties of the formed nano-grained particles are the motivation for this appeal.8, 9 In this respect SHS is quite challenging due to the high temperatures involved that lead to considerable grain coarsening in the product. Formation of nano-sized powder through SHS has been investigated by various groups.10, 11, 12, 13 For this purpose, effect of addition of diluents into the reactants, which are mostly preformed powder of the same kind as the expected products, has been investigated.13 However, the simple use of the product as a diluent did not provide sufficient grain refinement and the formation of submicron size particles is not achieved in most cases.12 The use of volatile species as diluent has been recently suggested as an alternative method. Specifically, in the case of the SHS leading to the formation of borides and carbides, NaCl has been employed.10, 11, 12 Nanometric TiB2 powder was reported to be produced through SHS using H3BO3, Mg and TiO2, and NaCl as the diluent.11 Grain refinement, due to addition of NaCl, has also been reported in the combustion synthesis of TiC from Ti and C powders.12 Starting powder mixtures containing H3BO3, ZrO2 and Mg have been used for the synthesis of ZrB2 by SHS and it was stated that addition of NaCl resulted in a decrease in the particle size of the formed ZrB2.10 However, utilization of oxide starting materials not only results in formation of side products like magnesium borates that require further removal steps such as acid leaching; but also decrease the efficiency of the reactions.7, 14 Additionally, residual ZrO2 remains in the products in spite of the precautions including utilization of sub-stoichiometric amount of ZrO2 in the starting mixture.15 Unreacted ZrO2 in the products cannot be removed from ZrB2 due to its insolubility in acid solutions.7, 15 Consequently, starting from elemental Zr and B powders was found to be more advantageous and is the topic of the present study.

In the present study, SHS preparation of nano-size ZrB2 powder has been investigated from Zr and B elemental starting powders with NaCl additions. Increased surface area of the nano-size powder obtained by this technique is expected to provide sintering at lower temperatures and also bring about better mechanical properties of the sintered parts, as compared to micron-scale powder.8

Section snippets

Experimental procedure

Preparation of ZrB2 from mixtures of elemental Zr (Alfa Aesar, 95%), elemental amorphous B (Alfa Aesar, 90%) and NaCl (Merck Chemicals) through SHS has been investigated. According to SEM observations Zr powder was composed of particles having 1–3 μm size and B particles were smaller than 1 μm.16 Starting materials in powder form were weighed in stoichiometric proportions according to reaction (1) then dry mixed and ground thoroughly in an agate mortar and pestle. NaCl (Merck Chemicals) was added

Results and discussion

Adiabatic temperatures of the Zr–B mixtures containing 0–50 wt.% NaCl according to reaction (1) were calculated by HSC software, and they are presented in Fig. 1. The calculated adiabatic temperature for the undiluted reaction (1) is equal to the ZrB2 melting point (3050 °C). The adiabatic temperature was not lowered for NaCl additions up to 10 wt.% although a decrease in the fraction of molten ZrB2 in the product should occur as well as a decrease in the actual ‘real temperature’ which is

Conclusion

Preparation of nano-size ZrB2 powder via SHS was demonstrated by adding 10–50 wt.% NaCl into Zr–B elemental starting powder. Reactions took place completely even with high NaCl content. Adiabatic temperature of reactions, reaction wave velocity, average crystallite size and particle size of the formed ZrB2 decreased significantly with increasing NaCl content. 30 wt.% NaCl addition was found to be the optimum and obtained ZrB2 particles were mostly finer than 200 nm. Hindrance of mass transport

Acknowledgements

HEC is grateful to the entire staff of Physicochemistry Department of University of Pavia for providing him the opportunity to study in their department as a visiting scientist, and for their help. His stay was funded by the State Planning Organization of Turkey via Academic Human Resources Program (ÖYP-DPT). Help of Prof. Dr. Ertuğrul Arpaç and Savaş Güven on particle size measurements is gratefully acknowledged. HEC also thanks to Akdeniz University Research Fund for financial support.

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Dipartimento di Chimica Fisica, Universita di Pavia, Viale Taramelli 16, 27100 Pavia, Italia.

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