Effect of grain size on the hardness and reactivity of plasma-sintered beryllium

doi:10.1016/j.jnucmat.2014.06.019

Journal of Nuclear Materials

Volume 453, Issues 1–3, October 2014, Pages 22-26

https://doi.org/10.1016/j.jnucmat.2014.06.019 Get rights and content

Abstract

Beryllium and its intermetallic compounds have attracted great attention as promising neutron multipliers in fusion reactors. In this study, mechanical and chemical properties of fabricated plasma-sintered beryllium (PS-Be) with different grain-sizes are investigated. Density and hardness analysis results of the fabricated PS-Be samples infer that a smaller grain size in the sintered Be indicates higher porosity and hardness. Sintered Be with a large grain size exhibits better resistance toward oxidation at 1273 K in dry air and at 1073 K in Ar/1% H₂O, since oxidation at the grain boundaries of the determines the rate. In contrast, at 1273 K in Ar/1% H₂O, a catastrophic oxidation is indicated by the increase of weight of the samples and the generation of H₂ from the bulk Be.

Introduction

Fusion reactors require functional materials, such as those used for generating tritium and as a neutron multiplier, to be loaded as in the form of pebbles in the planned water-cooled solid breeder concept. Beryllium (Be) and its intermetallic compounds have been extensively investigated as materials for neutron multipliers, and a variety aspects, such as mechanical properties [1], [2], reactivity [3], irradiation properties [4], and synthesis methods [5], [6] have been studied. However, the high brittleness of intermetallic beryllium compounds (i.e., beryllides) precludes their fabrication as pebbles and blocks. Nevertheless, researchers have fabricated beryllide pebbles (1 mm diameter) by a novel method that combines plasma sintering [7], [8] and rotating electrode methods [9]. Another drawback associated with beryllides is that Be in beryllides [10] is easily oxidized. Beryllium oxidation occurs predominantly at the grain boundaries; however, the presence of impurities can lead to the formation of porous and non-protective oxide layers. In case of pure metals, such as Ni [11] or Cu [12], diffusion across the grain boundary of the metal surface with smaller grain sizes results in a decrease of activation energy for oxidation of the metals. However, the influence of the grain boundary on the oxidation behavior of the beryllium is still unsatisfactory despite the reactivity of multipliers has increasingly become important in fields of not only the fusion reactor but a boron neutron capture therapy machine [13].

In this study, the effect of grain size on mechanical and chemical properties of plasma-sintered Be (PS-Be)at 1073 and 1273 K in dry air and in the presence of Ar gas/1% H₂O, respectively, has been investigated.

Section snippets

Experimental

Beryllium powders of various sizes were used to fabricate four PS-Be samples. First, Be powders (>99.5% pure) with four different particle sizes, <20 (Be₂₀), <45 (Be₄₅), <75 (Be₇₅), and <150 (Be₁₅₀) μm (Materion, USA), were prepared. The distribution in size was measured by a laser diffraction particle size analyzer (SALD-3100, Shimadzu, Japan). These powders were independently sintered in their plasma state under identical conditions to give the samples PS-Be₂₀, PS-Be₄₅, PS-Be₇₅, and PS-Be₁₅₀ (

Powder characterization and sinterability

To investigate the effect of grain-size on the properties of PS-Be, we first prepared Be powders with four different sizes. These powders are categorized on the basis of their size: <20 μm (Be₂₀), <45 μm (Be₄₅), <75 μm (Be₇₅), and <150 μm (Be₁₅₀). Size distribution profile and SEM images of Be₂₀, Be₄₅, Be₇₅, and Be₁₅₀ (Fig. 1) reveal that the particle size distribution, while being broad, are a clear distinguishing feature of the powders, while particle shape across different powders are similar.

Conclusions

The effect of grain size on the mechanical and chemical properties of plasma-sintered beryllium (PS-Be). Beryllium powders of different particle sizes, i.e., <20 μm (Be₂₀), <45 μm (Be₄₅), <75 μm (Be₇₅), and <150 μm (Be₁₅₀) are sintered under identical conditions to fabricate PS-Be₂₀, PS-Be₄₅, PS-Be₇₅, and PS-Be₁₅₀. Analysis of sinterability, hardness, and reactivity of Be lead to the following conclusions:

(1)
Synthesized PS-Be samples possess significantly high density. Samples with smaller grain size,

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It is also under study as interesting material for the future fusion power reactors. In the International Thermonuclear Experimental Reactor, Be constitutes the main plasma facing for the first wall of the tokamak and the neutron multiplier in the breeding blanket [6–10]. The use of beryllium in fission and fusion reactors will make it radioactive by neutron irradiation.
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Effect of grain size on the hardness and reactivity of plasma-sintered beryllium

Abstract

Introduction

Section snippets

Experimental

Powder characterization and sinterability

Conclusions

J. Alloys Compd.

J. Nucl. Mater.

Fus. Eng. Des.

J. Alloys Compd.

J. Nucl. Mater.

J. Alloys Compd.

J. Alloys Compd.

J. Nucl. Mater.

Metallography

Fus. Eng. Des.