Effect of nanometer-sized B powder on phase formation of polycrystalline MgB2

TL;DR

This study investigates how nanometer-sized boron powder influences the phase formation and superconducting properties of MgB2, revealing that B powder size affects sintering temperature limits but not phase formation temperatures.

Contribution

It demonstrates that B powder size does not alter MgB2 phase formation temperatures but significantly lowers the sintering temperature limit for superconductivity.

Findings

Phase formation temperatures are unaffected by B powder size.

Lower sintering temperature limit for NanoB-MgB2 compared to MicroB-MgB2.

Irreversible crystal structure transformation observed after high-temperature sintering.

Abstract

The size effect of the raw B powder on the MgB2 phase formation has been studied by the technique of in-situ high temperature resistivity (HT-rT) measurement. The onset temperature, Tonset, and the completion temperature, TPF, of the phase formation are determined directly during the ongoing thermal process. These two temperatures, Tonset and TPF of the sample synthesized using nanometer B and Mg powders (NanoB-MgB2) are 440 C and 490 C, respectively, the same as those of the sample using micrometer B and nanometer Mg powders (MicroB-MgB2). This indicates that the phase formation temperature of MgB2 do not depend on the B powder size. On the other hand, the upper limit of the sintering temperature, TN, above which the sample loses superconductivity, is below 750 C for NanoB-MgB2, much lower than 980 C for the MgB2 prepared using micron-sized B powder and millimeter sized Mg powder (DM-MgB2). In comparison with the sample directly sintered at 650 C < TN, an interesting, irreversible transformation in the crystal structure of the MgB2 phase was observed with the sample going through the stages of initial sintering at 750 C, then re-sintering at 650 C in an Mg-rich environment after the processes of regrinding and pressing. Possible explanation of the observed properties is discussed.