[Paper Review] Microstructure of the highly dense MgB2 Superconductor by transmission electron microscope

Gun Yong Sung, Sang Hyeob Kim+9 more|arXiv (Cornell University)|Feb 27, 2001

Superconductivity in MgB2 and Alloys3 citations

TL;DR

This study uses high-resolution transmission electron microscopy (HRTEM) to analyze the microstructure of MgB2 superconductor sintered under high temperature and high pressure (3 GPa), revealing a pore-free, 100%-dense polycrystalline structure with no intergranular impurities or defects. The HRTEM images show a defect-free basal plane with an a-axis lattice parameter of 0.307 nm—shorter than values from diffusion-processed samples—confirming high structural purity and ideal grain connectivity, which supports reliable transport property measurements.

ABSTRACT

The microstructure of the MgB2 superconductor sintered at high temperature under a high pressure of about 3 GPa was investigated by using a high-resolution transmission electron microscope (HRTEM). The TEM images did not show any pores in the specimen. All grains were compactly connected, and no discernable empty spaces or impurities at the boundaries existed over the regions investigated. The HRTEM image showed clearly each constituent atom that formed the basal hexagonal plane, without any defect in a single grain. The a-axis lattice parameter, 0.307 nm, from this direct measurement was shorter than the value, 0.314 nm obtained from samples prepared using diffusion techniques. A minor impurity phase, which was most probably MgB4 and did not form interfacial layers was also observed, but was well isolated from the main MgB2 phase. Our results verify that the MgB2 powder was sintered under high temperature and high pressure into its theoretical density without any porosity or grain growth.

Motivation & Objective

• To investigate the microstructure of highly dense MgB2 superconductor sintered under high pressure (3 GPa) and high temperature (900 °C) using high-resolution transmission electron microscopy (HRTEM).
• To determine the presence or absence of porosity, grain boundary impurities, or defects in the polycrystalline MgB2 microstructure.
• To measure the lattice parameters directly from HRTEM images and compare them with values from conventionally processed samples.
• To assess the structural quality and purity of the MgB2 sample for reliable transport property measurements.
• To identify and characterize minor impurity phases and their spatial relationship with the main MgB2 phase.

Proposed method

• High-pressure sintering of commercial MgB2 powder in a 12-mm cubic multi-anvil press at 3 GPa and 900 °C for 1 hour, followed by quenching.
• Preparation of TEM specimens via mechanical polishing to ~10 μm thickness, followed by 20-hour Ar ion milling at 5 kV and 0.6 mA using a Gatan dual ion miller.
• HRTEM imaging and selected area electron diffraction (SAED) using a JEOL JEM-2000EX microscope operating at 200 kV.
• Direct measurement of lattice parameters from HRTEM images, particularly the a-axis lattice spacing from the (001) basal plane.
• Analysis of bright-field TEM images and HRTEM images to detect porosity, grain boundary phases, and atomic-scale defects.
• Comparison of measured lattice parameters and microstructural features with values reported in the literature for diffusion-processed MgB2 samples.

Experimental results

Research questions

• RQ1 Does high-pressure sintering of MgB2 eliminate porosity and achieve theoretical density?
• RQ2 Are there intergranular impurities or secondary phases at grain boundaries in the high-density MgB2 microstructure?
• RQ3 What is the actual a-axis lattice parameter of MgB2 as measured directly from HRTEM images of a single grain?
• RQ4 How do the microstructural features of high-pressure-sintered MgB2 compare to those of diffusion-processed samples in terms of defect density and lattice parameters?
• RQ5 Are minor impurity phases such as MgB4 present, and are they isolated from the main MgB2 phase?

Key findings

• The MgB2 sample achieved theoretical density with no observable pores, confirming 100% densification after high-pressure sintering.
• No intergranular phases or impurities were detected at grain boundaries; all grain boundaries exhibited clean, defect-free interfaces similar to twin boundaries in YBa2Cu3Oy.
• The a-axis lattice parameter was directly measured as 0.307 nm from HRTEM images, which is shorter than the 0.314 nm value reported for diffusion-processed samples.
• A minor impurity phase, likely MgB4, was observed but was well isolated from the main MgB2 phase and did not form interfacial layers.
• The HRTEM image of a single grain showed a perfectly aligned atomic array with no defects in the basal plane, indicating high structural purity.
• The absence of porosity, grain boundary impurities, and lattice defects confirms that transport measurements on this sample are not compromised by scattering or inhomogeneities.