Mechanism of formation and peculiarities of structure of bulk MgB2 compound specimens

TL;DR

This paper investigates the formation process and microstructural features of bulk MgB2, revealing a two-phase structure caused by synthesis conditions and detailing the microstructural heterogeneity within the material.

Contribution

It provides new insights into the formation mechanism of MgB2, highlighting the existence of two distinct phases and their microstructural characteristics resulting from synthesis at high temperatures.

Findings

Identification of two phases with different Mg, B, and oxygen content

Microstructure consists of alternating regions 100-500 μm in size

Formation involves Mg melting, boron dissolution, and dendrite-like crystallization

Abstract

The existence of two phases within one and the same hexagonal lattice of MgB2 compound, differing in Mg and B (in the homogeneity region) and especially in impurity oxygen content, as well as in microstructure, is demonstrated by various techniques. The regions corresponding to these two phases of MgB2 have the sizes of 100-500 {\mu}m, and they fill the whole bulk of specimens, alternating with each other. It is suggested that the two-phase state of MgB2 compound is caused by specific features of its formation mechanism (as a result of synthesis at 800-1000{\deg}C), including the stages of Mg melting, dissolution of solid boron in it up to the composition of MgB2 and further crystallization of the MgB2 compound from the melt with the formation of dendrite-like structure with corresponding redistribution of main components and impurities.